Im going through the process of putting an 04 5.3 from a tahoe and a 4l80e into a 69 right now. There is a ton of good info on this site on fuel systems, motor mounts, ways to make gauges work with ls, etc. Here is a good link:
http://67-72chevytrucks.com/vboard/s...d.php?t=456787
The 04 5.3 is rated to 295 fwhp stock I believe. Have read on a bunch of guys getting to around 380 with just a cam and headers though.