I've heard High Performance Machine in Elko is good. Eddy hot tanked and sonic tested a 455 for me and seemed like a good dude. I think SilverBuick had him do some machine work and was happy with it.
Thanks Ravensbud, I took my engine to Herbert Performance here in Reno for cleaning and inspection. They have done some Buicks for guys here in town, one a 455 in a 1972 gsx convertible stage one, they have a very nice shop and seem very professional.