I've weighed a couple of lawn tractor batteries. They're between 15 and 20 lbs depending on brand. Probably won't make all that much difference for balance if your only moving it half way up the frame. But it's not too big of a deal to move it really. You can probably re-use your negative cable, then all you need is to extend the positive or get a longer cable somewhere.
I put mine way up in front. Just to get it out of the way for other things.
Had to weld up my own hold down. Depends on how much playing around you want to do.