Much cheaper to just get a junkyard diff with 3.73s already inside. It's a common ratio, shouldn't be difficult to track down.
This is the front differential, right? Is it the right style? 8.25 vs. 9.25? The 8.25 would be much more common, used on K1500s and light-duty K2500s.I found a 91 Chevy long bed 4x4 differential with 3.73 for 350
"I" would consider re-sealing it, verifying that the thermal actuator works, and at minimum, fresh gear lube before installing the thing. Converting to the newer-style actuator is a popular upgrade.