I run a red top 40 Rich.

Increased it last year as the 30 was struggling a little with the compression when hot.

Info here http://www.mdsbattery.co.uk/shop/pro...ctGroupID=1085

Got my old one knocking round if you need to try it. It wont fit in that box you have, but can get specific ones made for the red tops

**Edit** Just to say with these you dont necessarily need a battery box and can lay them down on their side etc