1500 nailed it, to put an lt1 in your truck, you'll have to mod what you've already got. nothing is compatible with the lt1, on your truck that is.
if you go procharger or boost, say bye bye to your gas mileage, if you go nos, you don't have the power on tap, you always have to fill the bottle and it's all go or none, with the ls1 swap, you'll have great gas mileage, awesome power on tap, and you'll still be just as fast as the other two, of course with a cam or so.

to swap out your engine, all you need is the longblock (i know you know already) ,

if you go with a built 5.3L, you still have less cubes, and how much are you going to have to mod that 5.3L to get an extra 70hp out of it.
you start off with more power going with the ls1,

good luck and i can't wait to see it run,

allen