From most of what I've read, the biggest gains are from better flow. In other words, the stock head got no flow.
There was a good article in Hot Rod about the Chevy six a few months back. It centered mainly on Sissell's Automotive
Sissell\'s and showed some interesting graphs of HP and torque gains from reworked head and fuel injection.
I'm thinkin' a 300hp 292 might not be all the streetable but, I hope I'm wrong. If you're Mr. Moneybags, just have Sissel's whip up a 292 for ya!
-David