I just wrote a long post in the interior forum on 3-point seat belt installs in a 3 window and it is about the same for lap belts but you can skip the part for the shoulder reinforcing plate.

Buy your belts and reinforcing plates on E-bay unless you can find them cheaper elsewhere, which I doubt. (E-bay seller TRUCK AND RV SUPPLY has great deals on belts.) The reinforcing plates come with the bolts you need for lap belts. You need four plates total for three lap belts.

I plug welded the reinforcing plates under the cab floor- Basically marked where the plates need to go, drilled a hole for the bolt and then one hole 1 1/4 inch on either side of the bolt hole. Bolt the plate to the floor and then fill the other two holes with weld. Grind, primer and paint.

One plate behind the seat riser at the each end of the tank and two inside the riser about 12 inches from each side. The 'tank end bolts' hold the tounges of the belts, the inside plates hold the 'clip-in ends' and both sides of the lap belt for the middle passanger, if you add one of those. Then the belts in the middle thread up through the space between the back and bottom of the seat.

I don't think you have the weld the reiforcing plates in place, especially if you aren't using the 3 point seatbelts. If you aren't welding, you don't have to take the tank out either.

I think the best place for the outside lap belt attachment points is on the floor at either end of the tank. Just be sure that when you drill the holes the are far enough back from the seat base so that the reinforcing plate will clear the structural reinforcement under the cab.

Rereading Caroline & Jims' posting I think you could also use the two rear cab mount bolts that come through the floor just aft of the door to attach the outside lap belt ends.

I can send you a link to the pictures of my install if you send me your e-mail address.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi