Damn i'd like one of those bad boys. I love the g-bodies.
Weren't those A-body? (Monte, GP, Cutlass, Regal) At least 'back then' (not sure if they were reissued as 'G' at some point - maybe when GM converted them all to FWD...sad!)
I saw one of these on Mecum a few weeks ago - the announcer was saying something about increasing boost (as well as naming the company where GM sent them for the 'Grand National' customization (like the seats with the stylized '6', the GN badges, etc.)
Supposedly, if you went to about 20lbs (stock was about 7lbs, IIRC) - that's when they became the 'beasts' they had the rep of being. I think there were a few aftermarket parts as well. Later, Buick came out with the GN
X, and also put this powerplant in the Regal T-type (those were grey / gunmetal, as I remember).
Damned expensive they were, too - when these came out, I had a license, but no $ - so I was driving a well-used F- or A-body, depending on the year. I finally have some coin - but not what they're asking for these two, and I want to relive my youth with a 1st-gen Monte, anyway.
As everyone else said - these were the dark days of performance. I think just before this came out,, a TransAm was the fastest car GM sold - faster than the Vette, if you can believe that. Might have been running the Olds 403 by that time - forget about a Poncho 400, much less the SD 455s that got dropped in '73 or so.
I also came across an article on the GN recently - talking about how these were the terrors (paraphrasing) of NASCAR, when they came out. I think they ran with the V6, not a V8 like everyone else had. If anyone is interested, let me know - I'll dig up the link.