or aft, of its mounting position.
Mounting an LS engine between the frame rails is only one part of the job; you also need to support the rear of the transmission. Although the LS bellhousing has an extra bolt at the top, its bellhousing pattern is the same as the Chevy small-block’s. This allows just about any traditional Chevy bolt-pattern bellhousing to bolt to an LS engine. Adapting the transmission mount to each vehicle is usually a combination of stock components modified with new mounts.
For most A-Body applications, the stock crossmember can be modified to fit late-model transmissions, including the T56 manual transmission and the 4L60E automatic. With the engine mounted to the frame, the transmission must be supported in front of the transmission mount for access. The transmission bolts to the stock mount if it is properly aligned with the engine.
The LT1 wet-sump pan does not fit the stock chassis. The only option at the time was to modify the chassis. Right before press time, a new aftermarket oil pan was introduced by BRP Hot Rods; Dirty Dingo is developing one as well.
You need to consider firewall clearance when adapting an older GM transmission to an LS engine and installing it in an A-Body. The Gen I small-block was designed with offset cylinder heads that leave about 2 inches of space between the bellhousing mounting pad and the back of the cylinder heads. Consider this spacing issue when deciding whether to use the stock transmission.
Some late-model transmissions are larger than classic transmissions. They sometimes do not fit the transmission tunnel or do not fit in the stock location. But there are solutions for achieving enough clearance. The automatics have removable bellhousings and these often take up more room due to the bolt flanges. This is the reason sliding the engine forward 2 inches is a good idea. With the engine forward, most late-model 4-speed autos fit without any mods, as do T56 6-speed manuals.
A-Body transmission crossmembers vary only by frame style. This tubular mount is used in most coupes, four-doors, and wagons. The crossmember has been hacked on and needs to be replaced, but it worked out for fitment in the 1969 Chevelle.
The Hooker LS swap crossmember works for all transmission types and is very clean and lightweight. This unit was used in the Chevelle for the 5.3/Muncie swap shown in this book.
GM A-Body frames have multiple mounting points built in. All you might need to do is slide the factory crossmember into another set of holes. Remember, an early transmission sits about 2 inches farther back from the LS engine, depending on the engine mounts. The LS has a flush casting in the back, and unlike the Gen I Chevy small-block, there is no extra material on the back of the block. This convertible/performance chassis is boxed from the factory for strength. The crossmember bolts to the top of the chassis in contrast to the lower channel with open frames.
Gen III/IV engines do not have offset cylinder heads and, therefore, the cylinder heads are flush with back of the block. When it comes to planning your swap, you need to adjust for this lack of space between the bellhousing and the cylinder heads. The cylinder heads are not necessarily longer, but the back of the block is shorter. Adapter plates for the stock location provide a space of about 2 inches between the back of the engine and the stock transmission in the stock location. In turn, it’s often necessary to relocate the transmission mount and/or move the transmission crossmember to bring the two components together, depending on the position of the motor mounts.
For stock-position adapter plates, the engine should match the same position for most GM automatic transmissions to fit in the stock location as well.
When choosing the transmission for your swap, carefully consider its location and how it will mount to the chassis. In the stock position, older GM transmissions such as Muncie 4-speeds, TH350/400s, and 2004Rs mount to their original location in the vehicle. For 1968–1972 cars, the stock crossmember fits and bolts into the correct location along the original nine-bolt pattern.
However, 1967-and-earlier cars have a four-bolt transmission crossmember pattern on the frame, limiting the options for the stock crossmember. Your engine position and transmission choice may require a modification to the factory crossmember or replacement with an aftermarket unit.
Another solution is the sliding transmission mount from G Force Performance. This special transmount bolts to the transmission and allows for up to 2 inches of travel fore and aft to reach the crossmember. These are particularly handy for swaps using sliding motor mounts; fewer mounting bolts means fewer options for mounting positions.
The late-model transmissions, including the manual Tremec T56 and GM 4L- and 6L-series automatics, are different. The T56 is a very long transmission, so fitting this into any A-Body with the stock setback requires fabricating a new transmission tunnel. The stock crossmember can be used in 1968–1972 A-Body cars with the stock setback adapters. Many owners, however, are not comfortable with or do not possess the skill set to fabricate a custom transmission tunnel. Cutting and welding in new sheet metal and chopping up the floorpan is a major undertaking, and many swappers are not interested in hacking up their car. The alternative is to use a forward-mount adapter with an aftermarket crossmember. This allows the T56 to fit with minimal tunnel modifications.
The key to correct transmission angles is to position the tailshaft at an angle between 2 and 5 degrees, and it should match the rear pinion upward angle. Too little or too much causes vibrations and premature joint failure. An angle finder should be used to set the transmission at the proper angle. The U-joints must be “working” to last; if the joints are positioned at 0 degrees, they will burn up.
For late-model automatics, you need to carefully consider the same factors. Here, the main issue is that they have bolt-on bellhousings, and that creates a clearance problem in the front of the transmission tunnel. The stock setback pushes the transmission back enough so that the bolt flange hits the tunnel. Serious modification is required to rectify the issue. Moving the engine forward 1 inch solves the problem, and the factory crossmember can be used as well.
When installing an LS engine, getting the driveline angle correct is critical in terms of strength and reliability. The transmission must be angled between 1 and 5 degrees downward on the yoke. For performance applications, 2 degrees is optimal. An angle finder (available at most hardware stores) can determine this angle. You place it against the tailshaft and let the needle rest until it points to the drive angle. If the stock crossmember bolts to the engine and the drive angle is between 1 and 5 degrees, it will work.
For an LT1 swap, the factory crossmember sits way too high, keeping the tailshaft from sitting at the correct angle. You could modify the crossmember or replace it with an aftermarket
