I just had a look at a battery supplier's page for various years and models, and they identified a swack of different Groups as being suitable. I suggest that you follow the approach we used decades ago when our Bugeye was our only car (summer and winter): measure the size of the hole, and buy the biggest battery that would fit in it. Make sure the posts were going to be the right place and that the height wasn't excessive. Maybe put a battery box in the hole first as well or something like an election campaign sign to provide a bit of a non-conducting, non-rusting layer.
I don't remember if the passenger side grab handle has a piece of wood behind the dash, but put one in there and attach a cigarette lighter/power take-off to the wood (so no worries about what to ground to) - we did this for our 3000. It's not visible, but can provide a GPS connection (or a phone connection, etc.)