I know back then when they wanted to jump HP, they liked to bump compression a lot. Look at a L88 427 and see how much power and compression it had vs a normal 427. The cams also probably have a good deal of duration difference. Probably not so much lift since valvetrains werent as stable back then.
**Edit** Ronnie beat me too it with the compression difference.
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5.3L LSx, Fbody .500 cam, TH400, 4500 Stall, Moser 9"
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