Also check locally for engine builders. Whether mail or order of local, you'll be asked how the car will be used, what your rear-endratio is, what kind of cam you want, etc.
What tranny do you plan to use? Mating a newer 6-speed to an older block requires a different rear seal, and not all the 6-speeds are the same (LT1 vs LS1). So I would try and think about that first. You don't want to receive your EG and then your tranny and find out they don't mate!
Owen