Yes it can be done. I suppose it depends on how much you want to do and how technically competant you are. (High degree of skill for this job.)
Recently, W. Roy Hunter (see below) and I have been working on just such a change. However, in our process we started with a body shell (no engine / trans / some of the interior) and were moving stuff from an 85 617 SD to a 603 SD. There are a number of issues you should be aware of:
Complete drive train removal and replacement takes a long time. So far about 3 weeks of part time work with 2 people.
We took everything from the engine to the entire rear end of the 617 SD and transplanted it to the 603 SD. In doing that the 603 SD 3 piece drive shaft was not able to be used. We removed the 300 SD driveshaft bracket and transplanted / secured it into the 603 SD. Then ground down the 2 other brackets. (rust proofing as we went along)
The rear dif is off-set and thus cannot be used with the 617 SD drive shaft / trans combination. The rear axle assembly including springs and support arms is a bolt in. But this is difficult to do.
The motor mounts are the same in both vehicles. I think the trans mounts are the same as well. (not sure, check with W. Hunter)
The engine shock on the driver's side uses a different bracket placement on the 603 SD vs the 617 300SD. (the 617 engine shock will need a new bracket welded onto the 603 SD engine cross member in the 617 SD location)
Electrical (starting ciruits) and vacuum systems are different and will have to be sorted out (a work in progress) You have to remember that the engine /trans was already out so we could not verify / save all the original connections. It's going to take some detective work.
The fore and aft false bulk head / heat shield on the passenger side can be disposed of. In its place you will need to install the heater pipe. The pipe uses the same holes as the false bulk head did.
Working on a Texas (southern) car is a lot easier than working on a rusty northern car (car we were transplanting to was from Texas).
Vacuum out all the mulch under the battery tray and brake booster or you may have a fire in your future.
The car is running, but not all electrical / vacuum are hooked up. The starting circuit is still a mystery. Roy had to jumper it to get it started.
Call / PM WHunter to get the real low-down and where he is at.
W. (Roy) Hunter (forum member & moderator and forum TECHNICAL SUPPORT Roy
Roy@PeachParts.com See the Fastlane left panel for a phone number.