I suppose you could put a BII body on a Ranger frame, but you'd have to cut a section out of the middle of the frame and shorten it up. Otherwise the wheels wouldn't line up with the fender openings.
Me and a buddy of mine are about to take on a similar process - we're mounting a full size Bronco body onto a 3/4 ton F-150 frame, so he'll have an '81 Bronco with 3/4 ton frame, suspension, and axles. The frame will be cut between the axles and welded back together with the appropriate Bronco wheelbase, the Bronco cab and fuel system installed.
I've tried to talk him into keeping the Bronco frame and just taking the 3/4 ton rear axle, keeping the 1/2 ton suspension and frame, because the leaf springs on the 250 have NO flex, but he's one of those guys that gets some crazy idea in their head and rejects all efforts of injecting common sense into the situation. I'm just gonna help so I can be there for entertainment value.