"Your Antique Value was lost the minute the original engine was replaced."
That's the truth.
And once it gets sanded and repainted too.
Sadly, they are only original 'once'.
"however, I do know that the truck I have is not a "find every day" so ... "
Come to Tennessee......and you may reconsider. They are in the 'Wheels and Deal' here almost weekly...mostly Chevrolets, but also a few GMC's pop up from time to time. Most unoriginal examples that need some fixing up are in the $2500 to $3500 range. The last real NICE one I saw was a couple of weeks ago...68 Chevrolet short step...58,000 original miles..I-6 w/3 speed on the column....one owner...garage kept...white paint almost like new....no rust...$5800. It lasted about 4 weeks, and then the ad was gone...so I don't know if it sold or not.
Also consider that Chevrolets seem to be more 'popular' with the old truck guys (no offense to the GMC guys...I like them too)....but they don't seem to bring what a Chevy will, at least in these parts.