Re: 572 or 427
You can make that much HP with a 427 but it will take really good after market heads, lots of compression and a large roller cam. It would be easier to make that much power with a 496 or 572. But even with a 572 it won't be easy to build 800HP N/A on pump gas. The pump gas 572 sold by Chevy is rated at 600HP and the race version is rated at 700HP.
What are your goals and budget for this car? With 700HP in a 3,400 pound car it should run low 10's or high 9's.
'69 Camaro - 429 SBC Dart Iron Eagle 9.325" block, Crower crank, Crower 6" billet rods, Ross pistons (10:1), Total Seal rings, AFR 245 heads, T&D steel body shaft rockers, Cloyes Timing set, custom ground 4-7 swap solid roller 274/286 @ .050" with .704" lift, Dart single plane, Holley Dominator EFI on E85, Injector Dynamics ID2000 Injectors, Stef's #1705R oil pan, F2 Procharger, Lemons 2" primary 4" collector, ATI 9" blower converter