Swapping from a 3-speed to an OD

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I recently bought a '85 GMC 2500 from a friend of mine. He bought it with a bad transmission and never got around to fixing it. It runs great and I want to use it for a work truck. I need to stop putting all those "towing" miles on my Suburban and stop using it as my work truck. Amazingly, it's still in great shape!

My question is that I have a recently rebuilt 700-R4 from a '85 Camaro that I was going to install in my '87 Monte Carlo. If I can use it for the 2500, I will. What are the problems I will be running into if I do the swap? My first guess is that the crossmember and tailhousing are different, the speedo will be off, and possibly a different driveshaft? I'm up for the task but I just want to hear from somebody who has done the swap or just knows whats involved. I will be doing some towing and possibly some heavy loads every once in awhile. I don't see having the TH-400 rebuilt if I already have an OD sitting in the garage. Also, any downfalls from having the OD? I just like the idea of lower RPM = better gas mileage. Thanks in advance.