I don't know Leon. I'm struggling with going parallel instead of series on the battery, but don't know the specs on your battery. With DC, you're not going to have any inductive current limiting, so not sure what the windings of the starter motor are going to see at 24 VDC (could get hot fast).
I was having the same symptoms, but different problem (voltage sag on cranking kept the LS2 sensors from reading any RPM during cranking). Anyway, I changed out the battery from an Optima, to a better, more expensive batter. Yeah, it's a beast in front of the passenger's feet, but since changing the battery to a larger one, I've never been left stranded after a stalled engine. Assuming with your usual quality work, you've got plenty of capacity in your ground and supply to the starter and ignition. Would a smaller generator pulley be helpful (faster spin at crank speed)?