Bucket Seat Swap - Help!!!

OK. I have found one of the "dents" the others are very difficult to locate in the positions where I need them.
The two outside dents - closest to the doors, I believe these should go through the bracket under the car to re-inforce the outside bolts. Then the stiffener plates must be welded in the "nests" provided to re-inforce the two inside (toward the hump) bolts - I think I have it now. Is there anything else I need to watch out for? I bought some 1/2" galvanized washers to shim the seat, once I get that far. Any good ideas to plug the old bolt holes?

Thanks for your help!