got to say the LS swap is good for fuel milage and power. Its no dog and even the lame ones are good power for street machine and are easily upgraded to more power later on down the road.
The real trick is finding the right deal. Avoid the demand for the corvette falvors they are too much money but the truck blocks are just as much fun. Best thing to do is research it and save your money for the day the deal comes along.
Check all the local junk yards. You will have your pick of engines to research. might as well get the 6 speed as well. It will help save on gas aswell. Not sure if the computer controlled auto tranny is as easy to swap but would also be pretty sweet.
Whatever you do dont spend 15K for ls7 427 swap. It would be freakin awesome but really not needed or worth the money since that kind of power is not impossible from the cheaper packages.