Love the 3b, still have one myself.
If your 6cyl is a v6 they fit quite nice,if its a staight 6 it might be a bit long but I have
never seen anyone try it.
Other things to consider,
-need new motor mounts or fit 6cyl mounts to frame
-transmision cross member location will most likely change
-drive shafts will need to be lengthened or shortened
-clutch linkage will change or use hydrolic clutch
Thats the first few things that pop into my head.
Would you keep original diffs or swap them too??
Let us know what you do, would like to see pics if you do the swap.