If you're really interested in doing a Buick, TA performance is about the only vendor I know. Kenne-Bell on the west coast was a Buick-only specialist for years, but have since moved into supercharging of all makes. IMO, a Buick 350 doesn't make much sense from a bang-for-the-buck standpoint. A 455 is no larger, no heavier, parts are the same price and more available, so you essentially have 100 extra cubes of torque with no downside. A few drawbacks: Performance parts are generally more expensive than Chevy parts due to fewer suppliers and much lower sales volume. Also, Buicks are notorious for oil pressure issues. The aluminum front cover serves as the housing for the oil pump gears, and wears with use, limiting oil pressure. I understand TA is now selling newly cast front covers, but I'm sure they're not cheap. The main bearing diameters are much larger than a typical Chevy. Combined with marginal oil supply ,the large mains make high revs a really risky venture. But Buicks are all about torque, tons of it, from idle up to about 4500 rpm. Build a mild 455 with a conservative cam, decent exhaust and you'll have a street motor that will make you happy. Having done a 455 for a friend's '37 Chevy, I applaud your intentions. Just do your homework before you make any moves.
Bill