My Deuce has a plastic battery box. I've read on the forum that Group 31 batteries fit fine in the metal battery box, but not in the plastic one. This is true, they won't from what I've seen. I also have a tmamerica 12V alternator setup on my deuce, so I need a separate 12V battery. A friend of mine works at Auto Zone, and I lent him the battery box to try to find a civvy battery that will fit 3 in the plastic battery box, and he did.
He was able to fit 3 Group 65 batteries in it. It has 850 CCA 1000 CA, and 150 min of reserve capacity. The batteries fit perfectly front to back and side to side. I think I'll only need to put a single strap across the length of the battery tray in the center to hold them down. Here's how it looks in the tray:


He was able to fit 3 Group 65 batteries in it. It has 850 CCA 1000 CA, and 150 min of reserve capacity. The batteries fit perfectly front to back and side to side. I think I'll only need to put a single strap across the length of the battery tray in the center to hold them down. Here's how it looks in the tray:

