I see now! It is the 10 lbs of $hit in a 5 lb box problem! Going with the V6 thinking have you ever considered the Ford/Yamaha V6. I think DOHC with 5 valves per cyl. Tracer Racing used one of these to set the E lakester record a few years ago.
The Chevrolet "small block" , 90 degree,based V6 is probably the best way to go, we ran a similar sized one with a set of hair dryers in our Lola T600 when I was at Interscope Racing. Ryan Falconer built them, used Porsche Kuglefisher injection from a 935 turbo and made about 900 hp, so the potential is there. We had some Chevrolet aluminum heads that were small block based and when we tried the V6 at Indy we had the splayed valve, big block style head. Those engines were making about 800 -850 at 9500 with 56 inches of boost on alky. I think that they were about 200 inches. ( May have been 160 inches, don't really remember)
Rex