This link shows step-by-step installation of shoulder belts in pre-66 trucks ...
http://www.bobshotrods.com/howto/3-p...chevy-pick-up/
I recently installed 2-point belts in my '64 and two things I recall that I want to share. If you locate the belts using factory specs, the hole you need to drill next to the driver and passenger door are located over a mount, so you'll have to drill through both the floor and the mount, which translates into longer grade 8 bolts, I think they were 4" long.
Second, be mindful of the gas line location under the truck, it's going to be really close to where you place the washer for the inner mounts.
A final thought, if you're installing a center belt, drill separate holes for them as well, don't share bolts.