If you think that every aspect was good for swapping then i dont understand your question, because obviously you didnt think this through because your problem was with noise but now ended up with mismatched reduction ratios.
This is why I asked why couldnt your mechanic overhaul that gearbox, its quite simple to do that, yes it costs quite a bit to do that compared to playing junkyard roulette, but the end result is good.
Now if you have no idea of any of the above and 100% rely on the mechanics you found (you claimed one mechanic, so im assuming you shopped around) - then bite the bullet and let him do as he says or find some proper one and have your truck redone by them. Like kaizrcruiser etc.
