75 and 76 factory part number books do list different part numbers for small block vs big block b-body k frames. Suspect other years will as well. As for what's actually different between them, couldn't tell ya and if there is in fact something different betting it would take meticulous measurement to figure out.
I've used boneyard donors from 73, 78 and 81 (original engines unknown) in to 75 and 76 small blocks. I've also dropped 440s in to 75s that I yanked the 318 out of. Even with keeping the original C body motor mounts they dropped in after swapping oil pans. If it's tweaked front/back side/side or diagonally I sure can't notice. As in: dropped right in to place like it was made for it and no fighting to get the holes lined up. Hood clears fine with OEM style intake / carb / filter, 440 fan fit in the 318 shroud, etc, etc.
Recall the same experience when putting the 318/904 in the C-body that was headed to the scrap yard after donating the 440/727. Dropped right in. Looked wrong as hell because of how small it was but again, no kinds of fighting to get the mount bolts in.
