You DO NOT attach seat belts to old truck seats or seat riser. You attach lap and shoulder using large thick plates. Under floor for lap. Near or on a structural member.
This is a VERY tricky issue. Because they were not designed in any way, shape or form for impact, you may actually be worse off with seat belts in an old truck. If you attach to the seat, the seat/seat bolts will break loose and seat will add to your inertia weight. If you attach to the floor and the seat breaks loose but belt holds, the belt will crush your guts. Modern seats and seat belts are designed to hold.
Then there is the shoulder belt portion. As you have been advised, the pillars are not designed for roll over, so the roof will crush you if you can't lean over because of seat belts....and the pillars won't hold a shoulder belt bolt. If the shoulder belt bolt breaks thru the pillar it will hit you in the head and kill you.
Have fun, drive slow, get good brakes and good brake lights.
