[Lost in the 60's]

I have this R3500 with a bad engine and am looking at a different cab/chassis to swap the roll back onto. The glove box insert has been replaced and there is no tag to indicate the wheelbase it was built with. I'm quite certain the frame was lengthened for the bed, but not the WB. I've tried the vin on all the free decoders on the www but none list the WB, just everything else about the truck that I already know.
Anyone have a source for a more thorough decoder ?
Thanks