A body motor mount

What about pulling my slant 6 out . mocking up my 400 b block and cutting the motor mount stands off and reweld them where i need them. 72 dart swinger . i never see or hear anyone doing this. So why not ?? I know there has to be s reason .
Because the slant 6 to big block conversion mounts are about 50 bucks. Trans Dapt 4697 is what you'll need.