I'm thinkin' that 650-700HP isn't gunna be any to bigga stretch with heads that flow in the range of 330cfm, 500ci and a tunnel ram. The reason being:
Early this year we built a 496.
.040" over 440 X 4.15 stroke
Ported Edelbrock Performer RPM heads 2.14/1.81 valves 305cfm @ .600, 321cfm @ .700
Solid roller 249/255 @ .050, .585/.600 lift
10.3:1 compression
Edelbrock Torker II intake
950 cfm single 4bbl
2" HVH carb spacer
91 octane
642 HP @ 5800 RPM/642 TQ @ 4600 RPM
The Indy heads as recommended earlier, tunnel ram and 500ci, wow, you've great potential there. For weekends, use a cam 262/266 @ .050, .696/.696 lift, maybe bigger. Sound like this is just for play anyway.
I think it is a good idea to stick to a 4.15/4.25 stroke to avoid installation issues with the bigger cranks.
In addition, just two weeks ago we dyno'd a 500 inch engine with 11.9:1 compression, ported 906 heads, Indy single plane intake, 1050 cfm 4150 carb and it made 742 HP on 91 octane. Big cam though, 280/288 @ .050", .816/.790 lift.
512 ci @ 650-700 HP? No problem.