LS conversions are becoming popular. I saw an ad for a 5.3 for only $800 locally. It had 108K miles on it. Read this:
http://www.hotrodlane.cc/77chevytruck/77chevy.htm
A TBI conversion is the simplest and least expensive. Stock parts are available from donor trucks or over the shelf. Read this for a TBI conversion.
http://users.sfo.com/~eagle/howell.html
For lower power grunt, TPI is good. TPI however has limitations on the top end.