i got 1991 the cheyenne w/t, and have done everything and more to convert it over to a silverado, and the engine is on my to-do list in a year hopefully when i have some funds saved
from what i know:
It is a fairly easy swap, extremely easy when compared to swaping engines on imports and stuff. the 350 has the same frame bolt locations as the 4.3 (252), and connects to the transmission and everything without having to buy new parts. i think the only thing u have to pull besides the ECU is the new intake manifold. also, if your cheyenne is older 252 like mine, and you're going to a 96+ 350, you're switching from the older chevy throttle body 252 to a Vortex 350, so I dont know if that requires any more modifications. i would suggest trying to find a 350 in the same year your cheyenne is to minimize problems.
also, another solution which ive always considered for my lifted 2x4
is to just do a body swap. find an old K1500 with a good frame and the 350 you're looking for, and swap the bodies, and sell the 'new' cheyenne and keep the now 4x4 cheyenne. there are only 6 bolts holding the entire body and cab setup on the frame for the 88-98 generations, so its not too hard