Swapping a Slant 6 for a V8

Like stated either conversion mounts or get a V8 K frame. You might want to start looking for a K-frame because Schumacher is having production problems and have been out of stock on most everything they make. I would use the 73-76 frame over the 68-72 frame. Better mounts and you can use the rest of you suspension parts.
I too would do a Magnum over an LA. Many upgrades including roller cam. I have a 400 hp 5.9 in my Dart and am really happy with it. A few million Magnums made so real easy to find.