on my mid 80s I cut both ends of the long bed floor to fit it in the short box.

tubs and gas fill bulge are in same spot. so measure from the edge of tubs front to back when chopping.

don't for get the frame needs a chop or swap to SWB

I swapped my 88 long frame for an 85 short frame so it was short and a 440 bolts in. repaired a short box.

more work but less body work on the slab sides.