A 2.78 ratio first and a 2.8l engine is not a good combination. He's going to need to ride the clutch to get the thing moving, it'll power out and stall trying to climb anything steep at low speeds, and it's going to have the exact same top speed. It makes no sense to put in a transmission that neuters the trucks performance.
The smaller the engine, the lower the gearing you need to make it drivable. He needs a deeper first, or deeper axle gearing to help that engine move the truck. Putting in a Mustang Toploader does exactly opposite of that. Same top gear, but now with a sucky first. Talk about putting in a lot of effort swapping and not seeing much result.