Dakota Seats n a Dart

I'm putting some Y2K vintage Dakota seats in my 64ragtop Dart. A previous owner had set the original seat bases back a good three or four inches, drilling new holes in the floorpan to do it. The Dart seat bases fit the Dakota seats with only minor modification, BUT the Dak seats are about three inches NARROWER than the Dart buckets, thus the need for more holes in
the floorpan. Compounding this is the fact that the floorpans are not flat, being higher toward the centerline, and sloping down toward the back to form the rear seat footwells.

Has anyone else swapped seats between vehicles and encountered similar challenges? It seems to be not really all that difficult to locate the seat with the bases installed and figure out from that where the new holes for the mounting bolts need to go. But I'm just a bit hesitant to go ahead it there's someone here with experience in this kind of project.

I'd really like to get some guidance in the tradition of the old adage: "Measure twice and cut once" This one seems like: "work the _heck_ out of the layout _before_ making new holes. Also, I suppose reinforcing the new holes with the largest fender washers I can locate should be adequate. Or do I need to use something thicker and or larger in diameter??? I guess the best way to close the old holes is to weld 'em, but I don't have a welder or any experience with one. I thought of putting bolts with fender washers and silicone sealant in the unused holes, but that seems kinda tacky. How about some two part epoxy putty worked through the holes and outward like rivit heads inside and out? Or pay a welder?

Advice - anyone???

Thanks in advance!

BC