Squeeze in the biggest batt poss.
Some nice previous owner of my 2-litre petrol (gas) 190E kindly fitted a huge diesel type battery, which just fits in. Two years down the line after buying the car at auction and no worries so far.
In my old '70 Minor Traveller's large battery bay (designed for the old type black 'rubber' batteries, I squeezed a nice big Mazda 626 battery. The little piddly new batteries die after only a year or two I find - keep your guarantee card in a safe place if you buy one cos you'll probably be needing it...