Ritchie, Phat is right on the target on this one. As long as there is no guaratee that the chassis is right, first thing to be done is check it all out. Assemble the truck body togheter all over the chassis. Only after You can make sure that everything is right in the correct place, the axles are in proper position, You should worry about the engine placemnet.
As for putting the engine way back is something that I do not agree completely. I always leave some room(like 2 inches of gap between the distributor housing and the firewall, so any mechanical job on it latter will easier, like removing the tranny only, just the engine or even removing the distributor) in order to avoid some future suffering. Of course that You'd be better off with a new radiator also, but it's not time to worry about it now. The driveshaft also, worry about it only when You're done in checking the chassis/body fit and after the engine/trans placement.
