Get the Optima redtop, then you can forget about battery issues for a long time.
Most autoparts places (Murray's, etc) carry two red tops, the red34/78 and a slightly smaller one.
The smaller one with fit in with no problem see the below pic.. the smaller one is sitting there:
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If you have a MIG welder and a cutoff wheel, I would lower the tray an inch and put then larger one in (I did that on a friend's snake).