After much research looking under many cars at Carlisle I did this in my 64 last summer.
Look under the car near the rocker panels. You will see some reinforcement brackets that the bench seat studs pass through. Look for another hole that is located in the bracket, not through the floor though. that is the location of the outer-most studs. drill through from the bottom in those locations.
Use your seat brackets to locate the inner stud locations. Make SURE the seat brackets are attached to the seats, with either both sides all the way back, or all the way forward when you mark the inner stud locations. The factory welded plates about 3-1/2" square on the top of the floor pan, 1/8" thick is good, at the inner holes only. Maybe 1//2" welds, 2 per side is good.
The seats are not symmetrical in the car. the passenger seat ends up being closer to the door.
AMD sells reinforcing plates if you don't want to make your own.
