I did the same thing on my 75 Dart Sport. It started out as a slant six car and I installed a big block using the Schumacher conversion mounts. Ran it that way a few years and then removed the big block when it wiped a flat tappet cam lobe. Big block is on hold for a while and had a 360 small block so I checked the Schumacher instructions for the slant six to small block conversion and it showed the same modification to the k-frame.