Finagled the stock seat belt receptacle to come through the hole in the seat (mostly. still a PITA to use). There was an empty threaded plate near the floor that I'm using. I assume it was the factory seat belt attachment point on early cars.
Patched the floor:
It's all sealed up and functional now. Hope to get the seat foam all squared away this weekend. Then I need to get an offset catalytic converter and weld it in and it should be good to go.