I'm actually doing something similar as you with my 54 3600. IFS, Explorer rear.
You shouldn't have any trouble with the Explorer rear. There's probably a few thousand vintage trucks running around with them.
You're right, the bed is the same width on the 3100 up to 3800 trucks. The big difference is in the frame. The 3600 trucks have a narrower frame in the rear and they are spring-over axle vs. spring under axle.
With the narrower axle, you'll have to go conservative on your wheel backspacing and tire width, unless you plan on tubbing the bed or widening fenders. I haven't got my rear installed yet, just the IFS, so I can't really help on what backspace you'll need.
