The '82 and newer cars had the 2004R as an option. This transmission required a mounting point farther back. The frame on the driver's side of these cars is boxed farther back with the corssmember mounting point back farther for the longer trannys. If you want to put one of these longer transmission's in a '78-'81 you have to extend the boxed section of the frame back farther. There are a few companies making these extensions with new crossmembers to mount the longer trannys.
Here is an example of ICEMAN's extension he sells:
It bolts into the original corssmember mounting holes and then you have to drill two holes in the frame to attach it at the rear point. At the rear of the extension there are two sets of holes. One set you bolt to the frame through the new holes you drilled and the inner set are to bolt up the crossmember.
ICEMAN's Maliburacing Page: http://www.maliburacing.com/iceman/