x2 with Pinion head. I don't think there is a "best place in America" recommendation because there are a number of great Pontiac engine rebuilders, both big shops and local shops, that would fit the bill. Each one of us can give a recommendation due to the experience and satisfied results with our own engines, but you may not be satisfied as each build is always different.
My recommendations in your search to narrow it down where you feel comfortable with a builder would be first select a shop that does Pontiac engines specifically. Many shops do mostly Chevy, Ford, or general rebuilding and may not have the knowledge to really know the fine points and ins-and-outs of a Pontiac engine.
Next I want a shop that builds race engines. These guys build engines to exacting tolerances and can't afford to do mediocre work or they would be out of business. It my cost extra for this level of builder, but you get what you pay for.
If you wanted to go a step further, find a shop that will not only build your engine turn-key, but put it on a dyno to break it in, dial it in, and provide a print out of your engines performance. No guessing left to chance and you know it will be a bolt-in and go engine.
Once you narrow it down, then I would make a list of questions/concerns and make my phone calls and speak directly with the shop and see how they respond.