If the truck is in equal/better mechanical condition AND has the long cab AND saves you from all the rust-fixing you were planning for the other truck, it sounds like you're not losing anything but 4WD. And that can be added more easily than doing a drive-train swap, right? Or you could do both, if you felt the need.
Would you rather deal with rust or 2WD? I would be jumping at this if that was the exact configuration that you wanted and you don't have much invested in the '78 anyway. You can add a front axle later if you want, but you can't add more doors to the '78.