Hey Jimmy,
Dimensions are 10.28" L x 7.06" W x 9.92" H (http://lifelinebatteries.com/products/marine-batteries/gpl-4ct/). An Odyssey PC2150 is 13.07" L x 6.91" W x 9.7" H. I don't have my '96 Discovery anymore, and never got to this modification, but it was my plan, and if I recall...