[ERA Chas]

I have a very early 427 car. The method in use at the time was an aluminum rectangle of floor material welded in place to the floor at the hole locations. No captive nuts either. The seat tracks thru-bolt with washers/nuts.
The harness strap ends go right into the frame with very beefy anchors. I also installed a sub strap with a gusset of my own. This whole thing sort of holds my a$$ in the car.
I have a buddy with a very recent FIA and will report his seat attach method.