Get the Olds. I got 521 hp out of a 403 without that much work. The cool thing about Olds engines is that most of the parts are interchangable (455 heads on your 350 if you wanted to). You can bolt on a set of heads with 2.07 intake valves, and 1.68 exhaust valves, with no mods. 2.10 & 1.71 are possible with minor mods.
Massive torque, too. If you like break-neck off the line power, and the ability to completely melt a brand new tire in one burn out, go Olds.
JMO.
Take care,
~Chris