this is a pretty good idea, I would add that after the piece is made weld a piece of 3/8'' bar stock along the edges to make it rigid, also be sure that the ends where it welds in are supported, as was mentioned before, there can be 10 gs on this in an harsh situation. Remember: strengthx2, adjustabilityx10

 

16 ga is extremely light for seatbelt mounting that's only the thickness of the floor. Would you lift up your truck by a bolt thru the floor? IMHO the plate should be no less than 10ga and then the mounting bolt backed with a large diameter grade 8 washer.
It's really easy to produce a shallow compound curve in a piece of sheet with an anvil and a ball peen hammer. Make a paper pattern of the piece using a stiff light cardboard like poster board or an old file folder. Mark the length and the width independently by curving the pattern stock first in one direction and marking it for size, then in the other direction, marking that direction. Lay out the pattern on a piece of cold rolled steel adding about 1/4" all the way around for "wiggle" (trim) room.
Cut it out then with a permanent marker draw a line around it about a 1/2" inside the edge. Outside that line is "no man's land" where the sheet will not be disturbed. Now lay the piece of sheet marked side (which should be the inside of the compound curve) up on an anvil, I beam, section of railroad track, or other heavy smooth piece of steel. Using the ball end of a good sized (16-20 oz or larger) ball peen hammer give the piece of sheet a hit right in the center just hard enough to leave a visible impression ~ 1/4" in diameter. Using that same force begin hammering in a tight spiral from that mark, overlapping the previous mark ~ 1/4 of the way, there should not be any unmarked surface between impressions. Continue hammering in this tight overlapping spiral pattern until you get to the boundary of No mans land. If you have done this correctly and evenly the edges of the piece of sheet should be starting to curl up into a shallow bowl shape, the beginning of your compound curve! if you need a deeper curve repeat the process of hammering in a spiral as many times as necessary. NOTE: It's consistant light overlapping blows that will produce a smooth compound curve be patient, heavy random blows will just result in a useless bent up piece of metal. If the curve needs to bend more in one direction than the other, use the above method until you have reached the shallower curve all over, then turn the bowl upside down over a domed form like a body dolly or even a large steel ball bearing welded to a heavy rod you can clamp in a vise. Use the flat face of the hammer to hit the metal directly over the support and move it a small amount and repeat in a linear pattern parallel to the direction you want the metal to curve, again avoiding no man's land. If you want to practice first cut a piece of plate ~ 4" square or a disk that size and see if you can make a nice even bowl shape out of it.

 

Ax is right about the thickness of the metal. I am only guessing that it was 16 gauge. It might have been 10 gauge. (My memory is shot) Thicker is better as long as you are talented enough to weld the two different thicknesses of metal. But I would say that 1/8" is prob overkill. The ones that they sell thru mid-fifty I know are not anywhere near that thick. Also, I believe that the installation process on the 53-56 is stronger because of the placement of the weld-in points. I might be wrong, but my understanding is that the pre-53 cabs only give you two points to weld to while the 53-56 cabs have three. Remember to weld that 7/16" fine-thread nut behind the plate. That means that it would have to pull through the plate before it failed in a colision. Good luck, Jag