Couple of thoughts: 106" wheelbase (kinda short), COE (kinda tall), '75 half-ton suspension (kinda clunky for lack of a better term)...sounds a little tricky to make work. Wish there was a pic.
I would wonder about the weight distribution, is it all on the front? A very light rear end might be enough to make it wander around some...I'm not sure how I would address that other than to make sure nothing's loose back there, that if it's a custom suspension is has some roll understeer designed into the geometry (i.e. if it's leaf-spring type, they are angled forward a little...or if the airbags are working alone, the suspension links f-rr are level or angled slightly up toward the rr) then experiment with tire pressures (more in front, less in back and just to see what happens vice-versa) and even load a couple parking stops or something heavy into the rr of the bed just to see what happens. I know it seems like I am working from the wrong end here, but I'm trying to consider more than what's already been looked at. If the front and rear tires are radically different in size or sidewall stiffness it would be a factor. Even frame twist, with a heavy high-C/G front and light rear, could be messing with you.
Another matter about custom suspension, is the front Chevy suspension mounted level to the road, as seen from the side (lower a-arm cross-shaft same height from the road f&rr)? If it's level to the frame and the truck has a lot of forward rake, that could add to the problem.
What front wheels? Do they have a lot of positive offset? The drop spindles may already place the wheels an inch-or-so out from stock, add more with wide or custom wheels and it's going to wander around, especially if heavy at the front. Only solution there (if that were the problem) is narrower wheels or otherwise bringing the offset (as measured from center of tread) in.
Negative camber is for sports cars, trucks should have a little positive...that could also be a factor but I can't say what exactly would happen with changes, just something to throw in.
Is there any kind of a bump steer situation happening? Are the lower a-arms about level as seen from the front, and the tie rods parallel with that (compare with a-arm pivot point centers, not component shape).
I would normally up that caster setting to around four degrees positive but the general rule is, heavy at the front = less caster, cars that are light up front get more. So I'm not sure what I'd do other than to experiment. Try four, don't bother with trying different settings left/right at this point.
Considering all things especially how weird your total combination might be (I use the term "weird" in a good way), the best thing to do might be to have a really experienced chassis guy sit in the driver's seat and try it himself. Your average specification-reader down at the corner alignment shop isn't going to be able to help much when the total package deviates too far from stock.