If you're going through all that trouble, what would be wrong with dropping your body mounts and reworking some of the trans tunnel? The frames on the 73-87 are fairly tall, and suck up nicely inside the body, so, with a little modification, the newer fame should too... The body mounts probably won't line up right anyways, so dropping and moving them shouldn't be too hard. I'm also assuming there is a gap between the bed and frame, as well as room to lower the core support mounts...essentially, you'd be body dropping the truck a little...lots of work, but so is the drivetrain swap...