It won't "hurt" to use a slightly lower capacity battery but you may find it will go flat on you more often (with a high accessory load).
I'm using a
DIN88 battery with is rated at 860 CCA and 100AH - it's pretty much the largest battery you can fit in the space. A
DIN85 battery was in there prior to this one (780 CCA 86AH) - worked fine in my 3.0d.