the oil pressure gauge is not about your oil level. (unless the level gets too low and the pick up tube doesn't get to the oil) It measures the oil pressure that the pump is able to produce. Things that affect oil pressure are the weight of the oil used, temperature of the engine, engine rpm, clearances in the oil pump, the screen on the oil pick up, pressure relief valve, cam and crank bearing clearances.
thicker oil raises the oil pressure. higher engine rpm raises the oil pressure. tight clearances in the oil pump increases pressure. same with bearings. Just from the information you posted, it would seem your mechanic
has made a logical call on replacing the engine or crank kit it.
I am sorry that I don't have any good news to give you. You could see the place where you bought the truck to see if they are willing to help you out. If it's a used car dealer, see if they would take it back on a trade. Or try to sell it and buy another vehicle.