Just cut the bottom out and rip the innards out. It's hard work. (Obviously you need to rinse the battery out good first) Up at the top there is a bunch of lead chunks that connect everything together up there. Cut all that away leaving just the posts. Be carefull not to knock the posts loose. Then drill and tap a couple holes in the bottom of the posts. Then bolt a couple of short battery cables to the bottom of the posts. THen put your little battery inside it and hook it up to the cables. I use a Dyna-Batt http://www.performancedistributors.com/batteries.htm I use the one with bolts on top, not batery terminals, then use battery cables with an eye on each end. THe foam packing thet the Dynabatt is packed in for shipping fits nice between the old battery case and the dyna batt to hold it in place.

Have fun!