Can I put crew cab on 1994 gmc regular cab frame
I think so but use the shorter crew cab bed too. Or weld-in a frame extension.
there was no shorter "crew cab bed" in 1994. you will have to stretch the frame, wiring, driveline, fuel lines, brake lines, and probably a few other things I am forgetting.
And Or cut & modifiy the bed? I saw this done on TV once. A body shop is recomemed, They cut the end off the rear with the tail gate off a long bed, removed a 12-16" or so section aft of the wheel wells, and welded it all back togeather. Wal-la, a custom short bed! Much easier than cutting up your frame, brake lines, fuel lines, drive shaft, wiring, etc. Formula: What space or length you gain from the crew cab, must be taken out of the bed.
OR don't have a bed? Just add a flat frame with wood planks & stake body sides.
Us old timers used to make everything! Not many off-the-shelf or factory parts availible. Be creative.
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