Slant 6 - 727
I've got one in my truck. No adapter or anything. Seems odd for Dodge to offer a 727 and an 8 3/4 axle behind a lowly slant 6. My Dart has the same trans and axle behind a 408 and it's been solid.
I recently met a guy with a hoard of 70's and 80's Dodge trucks. I pulled a manifold off one last weekend and it had a 727. I bet he would let it go cheap if anyone needs one in Oklahoma.