The ammo is this: these big old trucks have a very small following, odds that he'll be able to part it out without putting it on the internet bit by bit are slim to none. As for the trans or rear end being something special, due to size, weight, and shipping issues, they'd have to go to local buyers & as such he's less likely to sell them at all, if ever. Same for the odd sized body panels specific to the big bolts.
I've seen guys be difficult with junk yard rigs just because they dont really want to sell them, so it's going to be a crap shoot either way.
I'd find out what the scrap value is (current $ per ton) and offer him something respectable.

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P