Small block to big block trans cross member doesn't work after swap?

I had the same issue when I put a later model 318 and 727 from a tradesman 200 van in a 67 B'cuda that was originally slant 6.
The 67 A-body was still much like the 1st gen' A-body in a few places. That cross member wasn't relieved on both sides for dual exhaust pipes, and the 727 hit the panel seam. Trans mount didn't line up either. My experience was back in 1982 so I dont exactly remember everything I did. I recall air hammer to fold that seam away. A different cross member, It still wasn't the spool mount type cross member. I might have tuned it around to make the mount line up with that trans and hogged out the bolt holes.
For what its worth, factory went through the same chit to put a big block and 727 in the 67 model. The car came off the assembly line and went to a seperate bay so the engine and trans could be hammered in. All of this was sorted so a-body could easier get a big block and dual exhaust in 68 up. Good luck with yours.