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Like said above, either brand will work, but just ensure that you're getting the same gasket thickness that's under the other head. I used Felpro a couple months ago and haven't had any issues.
I'm pretty sure there is more than one dealer, so that's a tough question to answer...
Some charge list price (major $$), some will charge wholesale if you schmooze them, etc.
The list price is pretty much the same no matter what GM dealer you go to. The wholesale price may differ as some give better deals than others, just depends on how much business you do with them.