This is a pretty common problem we have to deal with in the old car community whether or not it was originally delivered in OD paint. If you look at the attachment points for the seat belt mounting buckles you'll note that they're always backed with a welded heavy plate and nut to prevent pulling through the sheet metal of the floor pan. Car companies weld these in now, but in the ~5 year period when seat belts were an available option on cars before they became mandatory, a seat belt kit included a big (typically 2" dia) round backing washer of about 1/8" thickness along with the expected Gr. 5+ bolt, nut and lockwasher. So you're going to have to look under the body for likely mounting locations where you're a) willing to drill a 3/4" hole, and b) there's room for the backing washer in c) about the right place. Don't worry too much about ideal placement - the car manufacturers surely didn't - they tended to optimize for finding good and convenient structural locations rather than putting the hole in the ergonomically perfect spot. But without that backing washer to spread the load your seatbelts will pull right out of the floor pan if they're ever needed.
As for what belts to use, an automotive seatbelt after about 1972 is like a motorcycle crash helmet: a single-use item. This is not for economy, but by design - you want the seat belt to absorb the energy of collision (by stretching) rather than imparting the energy to the human wearing it. They work really well for this purpose - once. So always buy new (or new old stock). The M998 belts meet the standard and are designed to be mounted to a flat panel like you're dealing with. I would be cautious about using belts from things like armor or aircraft (even though they're often OD) because they're more like racecar harnesses and don't work well unless they're fitted just so to the body they're strapped around.
See
this thread for information on how a similar unit-body truck of the same era was fitted for seat belts. There are drilling instructions, locations, and mounting examples from the factory assembly manuals scattered through the thread.
