Initially, I think I would shoot for about 36 total in at about 2800-3000 rpm (it's hard to say what the truck will like, but I think thats where I would start.)

Also keep in mind that when you are pulling away from a stop sign (under load when actually driving instead of sitting stationary in the garage), you intake vacume will drop giving little or no vacume advance. To trouble shoot it I think I would plug the vacume advance temporally (to insure you don't get too much advance when the vacume raises at cruse speed) and add some initial timing and see if that helps.

My mind also keeps going back to your comment about running better with the longer reach plugs. Were asuming that you have the original heads on the truck, but are you sure they are? I think I would pull a plug that easy to get to and see if I could tell what the depth is to insure that the electrode is actually in the combustion chamber. (Another way to do this would be to pull a valve cover and run the casting number to see if the heads are actually correct for that engine.) With the engine combination you have, I would start with the plug that the engine the heads came off call for + 1 step hotter.