engine question

Well, I have a simple hypothetical question for you guys. I have no desire to really do this any time soon, but am curious about the possiblity if in say 10 years or so I still have the truck and the engine blows out. I have the 04 silverado reg cab w/ 4.3. Is it possible to swap out the 4.3 to one of the larger v8's without too much trouble or expense beyond normal? I can't remember which gear ratio it has in it right now, but do remember there were two options for it and mine is the ratio that will do the heavier towing up to I think 5,000 lbs or so.

Also, I am curious about the locking rear end. I know it was offered for these models but mine doesn't have it. Is it relatively easy to add? I almost got stuck in a parking deck here in raleigh when we had an ice storm last year. I had to stop on an incline for the car in front of me and when it came time to go again all I could get was one wheel to spin. Had to get some guys to push. I know some sand bags or cinder blocks would help, but wonder about the locking rear differential.

Thanks