GMC's are considerably rarer, and the custom trim even more so.
I'm lucky in that my 63 has pretty much the whole shebang regarding custom options; cigar lighter, clock, big back window, front over-riders, 4-way flashers.
If yours has the 305 V-6 engine then it's rarer still, as many people junked them and fitted V-8's. Nothing wrong with the 305 engine though.
I tell people that GMC's are a "gentleman's" truck, Chevys are for the "common herd"!

(Puts on tin hat and flak-jacket awaiting incoming!

)