With all of the gearing you're going to have with those portals I'd probably just do a N/A 4cyl like a 1UZ from a toyota or even a 1.6L Tracker motor, if you want a V6 and want to stick with the a GM transmission a 3.6/3.8 Pontiac or 4.3 GM. The portals are going to make you geared really low, and using a Sami t-case if you regear it you'll also be underdriving the high range so you'll want something that can rev and let you have a decent top speed between trails/obstacles. You won't need a lot of power down low really. It seems like the majority of the lightweight portal builds I've seen are running ecotecs, 30RH's, and dana 300's to keep weight down as much as possible. The advantage of the portals is that it moves the torque load down stream so the small Dana 300 holds up better and a lot of times doesn't need a 4:1 kit installed.