I've got a 78 Jimmy. The body's rusted to kingdom come but the frame and drivetrain is in great shape. I found an immaculate body shell from an 89. I know that GM came out with the GMT400 platform before 89. My question is: will this body fit on my existing frame? The bodies look similar in shape, but I don't know if the bodies will interchange due to the platform swap. Thanks in advance!

 

I believe that the body mount locations were changed somewhere between 78 and 89. If you were to look up the body mounts in LMC Truck's catalog, you should be able to find the year break.

It isn't impossible to do, but I do not believe it will be a direct swap.