I once picked up a load of w cedar siding for the cabin using a E150 Ford van.
This van had an aftermarket 2ed seat/bed unit that folded flat, allowing wood to be stacked both below and above it. The back was a "table" about 15" off the bottom that gave a flat smooth surface across the whole back.
Anyway, we loaded it up and had about 1,000 BF of 1x6 siding inside, 8's, 10' and 12's.
The wood was mostly solid from top to bottom, side to side, and from windshield to backdoors, except for the drivers seat, a small void where my young son and the dog squeezed in. His bike got strapped on the outside.
I'm not sure if this load sprung the rear springs or another load of gravel/rock did
The second load of cedar interior siding didn't have quite so much inside as we also used a trailer that trip. This was about 1,400 BF of 1x6.
On both trips we drove very slow, maybe 30-40 mph for the 25-30 miles back to the cabin.
Or to answer your questions, if you are getting 100 inch pieces, and you stack it high, maybe 300 to 500 BF based on what we had in the van, which was stacked both higher and longer then you would in a 1/2 ton truck.
Or you could also calcuate it based on the sizes of the pieces you are getting.
Dan
(who really likes white cedar)