Johnny- that sounds like a REAL sportscar there... havin' dual batteries over the axle means you'll actually get to stick to the road a little more with the fun-pedal depressed a bit to 'religously'..
Brian- My little boat has a pretty viscious Chevy inline six... LOTS of compression, and dual-batteries. I run 2/0 marine battery cable for both, and the cables are only 2' (grounds) and 3.5' (each positive to battery switch) and a 3' cable to the switch. Changed all my Jeeps over to 2/0 for ground and power, too.
I think the stock IHCC cables are #6, I used #2, and my grounds are direct to the engine-block. I'm pretty certain that the slow-cranking problem, however, is a combination of two things- first being cold HyTran, second being enough wear in the end-bushing of the starter-generator to cause one of the brushes to not seat properly.
By not seating against the commutator, it doesn't matter HOW much current I make available at the battery, or at the S/G terminal, the SG's motoring-mode (a series-wound configuration) will be current-limited in both field and armature, by the brushes. When I hook on a 12v booster battery DIRECT to the SG with 3' of 2/0 jumper cables, it doesn't turn any faster than when cranking with the on-board battery... it'll just crank slow for LONGER. When I series two 12's, it kicks a$$ and takes names.
I'll be opening up this SG in a day or so- I'll be able to see for sure then!