I would consider an 6.0-LS 4L80E swap over a 350or454 700R4 personally.
Jack, how much are you looking at spending ballpark for engine and trans? You seem to be leaning towards a crate motor, but like Justin said, are you really going to be putting the enough miles on this thing to justify a brand new crate motor(sounds like this is going to be a trail rig)? The reason I bring this up is because you can get a 6.0L "LS" motor for pretty cheap as well as the 4L80E. Even if it has 100K on the clock, it's still going to last longer than a brand new "vintage" 350 design. I would think you could get the newer engine and trans for less than a new crate motor to boot, and they typically start out with more power anyways. The only thing you'd really have to do would be get motor mount adapters, and they are very cheap and literally bolt on. The newer engines already have fuel injection and better fuel management too, so you really save in the long run.