I used the 3 inch wide seat belts because I figured the wider the webbing the better. Plus it's easier on your body. Use a piece of rope to ascertain the length you need,taking into consideration different girth of passengers and what length is best when seat is all the way back. Duct tape the rope in place to find out what length you need.
I drilled my holes in the floor pan in an open space behind the seat with the front seats all the way back. You don't want to drill into the frame , muffler, etc. DO NOT ATTACH SEAT BELTS TO CAR FRAME.
Buy from a company that supplies the mounting hardware. I found it best to use longer belts as you can always adjust them shorter.
Here's one company: There are others.
https://www.wescoperformance.com/3inlapsebewi.html