I don't know enough about G3 Hemis, if they can make 1200 boosted with stock block and heads, that would be really hard to beat.

Turbos and E85 at a minimum on a moderate sized RB would be my route, but I'm a big block person. The G2 hemi chambers are better for boost than wedges but also have heavier valvetrains and would be harder to package a turbo kit if you're trying to keep inner fenders. I don't see many huge CI twin turbo setups, it seems 540-555 is pretty common, with 572 being the upper limit. At "only" 1200 hp I'd think you could easily get it done under 500". You also wouldn't need the best of the best for cylinder heads either. I know of a couple 440-1 headed turbo big blocks that make 1200+.

One thing I have seen interesting is the higher horsepower turbo BBC stuff sounds like they actually move to our Chrysler wrist pin and rod journal size for additional load capability.

Bigger concern is I don't think a 727 is going to live at 1200 hp behind anything long term, so a turbo 400 would be in the cards too.