I'd stick with the original set up for simplicitys sake IMO. Just upgrade to a 350 ci, or 383 for that matter. The tbi set up may not be as "cool" as the LT1s mpfi, but it's dead reliable, cheap, and simple to fix if somthing goes wrong. Get a 350 tbi long block, 350 injectors, computer, h.p. cam, tb spacer, and low restriction exhaust and that wagon will wake right up. There are after market intakes to put '96-'02 sb vortec heads on tbi engines which are good, relatively cheap upgrades. Edelbrock has a kit to convert tbi to mpfi that a former co-worker of mine has (or had) on his '93 or '94 350 SS C1500 p/u, and that truck moved when he did that conversion along with a cam change. I've seen the Edelbrock set ups pop up on ebay once and a while for $300-500. As for the Nissan, trade it for a Chevy. Ha ha
Good Luck, and have fun with it.
Mike