[padresag]

there are other probabilities. does the frame # match the cab vin #. the vin # should describe the w/b. some one could have put those 3/4 t axles under it and and a 3/4 t cab. I changed to the heavier axles under my 64 Gmc 4 x 4 panel, sme w/base as yours. in over 50 years things can get changed around. people do a lot of strange things, but as a 3/4t 4 x 4 I am not saying that it is not impossible but perhaps highly improbable. it would really be interesting to see pictures of both sets of vin nbrs.
ron