Final configuration:

15 (3cells X 5 blocks) on both sides of the motor, 20 (4 cells X 5 blocks) in the gas tank slot. The blocks are strapped together to make handling the cells easier. Had to buy the strapping tools and strap/clamps.

That leaves 6 cells as spares, although I might use 4 for the 12V battery, just so they are on board if I need to swap any out. Too bad only 3 fit on the original battery tray, I'll have to cut it or make a spacer plate, as I don't want to have them separated with wire in between. (4 cells makes 12.8V, 5 would be 16.0V. )

Redid my relay board to look more like the original, losing the watertite box all the relays were crammed into. Got some ABS sheet and made a cap for it, getting it mounted today.