Battery cases crack on older batteries because of expansion of the positive plates, in time it may bulge enough to short out but until then if you catch it quickly it's redeemable.
I use auto bog to repair the crack, refill with new acid and put it back to work, if you do not have acid on hand, refill with distilled water to preserve the plates until you source some acid.
I live with old batteries many now with repaired cases, my feeling is that the larger battery will be OK ( not normally recommended) just keep an eye on the SG (specific gravity) you may need to equalize for a little longer than normal.
allan down under