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.
I currently have a 360 LA engine with Schumacher slant six to small block V8 conversion mounts running and I did not have to further modify or reverse any of the modifications on the k-frame. Schumacher is currently up and running under the new ownership, so I would just get the small block conversion mounts and bolt it in.