I have a 75 Dart Sport that was originally a slant six car. I used Schumacher slant six to B/RB conversion mounts to put a low deck big block in it. Was run that way for a few years. Two years ago I put a 360 small block in with Schumacher slant six to small block conversion mounts. Both conversion mount kits used the same k-frame modification on driver side, so the small block went right in since I already previously modified that k-frame.
I can’t think of anything that I would consider a drawback to using those mounts, unless you make enough power that an engine plate would be a better choice.
I used the Doug’s D453R headers because I was in a hurry. Fit was tight around the steering coupler on driver side. I forget if I dinged these headers around the torsion bars or if that was the old set I was originally trying to fit. Also, mine has Edelbrock Magnum heads and the block was decked some, so the headers may not be in the intended position on this one.