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My advice is to pick out a small local speed shop. Buy all your "speed" from him, even though you can get the same parts from a catalog for less. Find a guy who isn't just trying to sell you as much stuff as he can. Make friends with the guy.
Then let him advise you where to get your engine rebuilt. Someone who has been doing business in the area for years probably knows who is and who is not competent.
This worked for me; no leaks.
In any event, I advise you to either go local or go GM. If you buy a crate engine by mail and the engine dies, getting warranty service is a pain. A local engine builder will be available to honor his warranty. GM offers a nationwide warranty and nationwide service.