I am thinking I could use the voltage doubler in the Toyota inverter. I will just be providing a compatible battery voltage to the input. Toyota states 288V nominal for the Highlander, and I will be right there. The Soliton1 controller can accept up to 340V max.
I think the C rates you are quoting are for the Enerdel power cells. I will be using their energy cells, so the C rate drops to ~2C continuous, and 5C peak, though I have seen characterization which shows these rates to be somewhat conservative. At 6P I will already see a per 'cell' impedance drop from 0.7mOhm on the old Thunderskys to ~0.45mOhm.
Here's a basic floorplan showing old pack vs new pack, and a placeholder for the MGR. Mounting it in reverse would be the best way, as mounting it normally would require putting the batteries out beyond the rear axle line.