It's been a while since I last drove the '62 GMC 6500 I used to drive for a company (1974), but I seem to remember the low floor in that truck too. I also bought a '62 GMC 4000 in the early 80's and same thing, I remember it having a low floor. Now they are the same cabs as a pick-up, as also mentioned above, but a big bolt it is way up higher and you have the running board and fender fillers that go down to it, therefore there is a lot more room under there and no need for a large floor hump.
Scott, the 3-Speed floor, as I mentioned earlier, has a low hump and the 4-Speed trucks (1/2 thru 1 tons) have about 5 or 6 inch hump. It would be worth a try to see if a 3-Speed mat would fit in a big bolt. Instead of chancing a new one, try to buy a decent used one from a junk yard. Just cut out your own holes for the shifter, emergency brake, PTO, etc.
Bill
