I've been thinking of putting some new seats into my car. It's a 1970 Cutlass S that probably had a bench in it to begin with. It's currently got a pair of buckets out of God only knows what that the PO never drilled the two inside holes for. I'm thinking of getting the front seat out of a 2001-up Caddy Seville with the integral shoulder belts. I measured one in a friends car at 52" wide at the bottom and 54" wide at the top so it looks like it should fit inside the doors. What would be the best way to support the seats in the floor pans? Drill through for the bolts to locate everything and then weld a piece of U channel along the bottom and drill the bolt bolt holes for it too?

 

This thread should help ya!
https://classicoldsmobile.com/forums...ern-seats.html

It sounds like your mounting method is on the right track.