Consider 6V in series
Bob, I had a similiar problem on my Beneteau 361. My single 4D had seen better days and I wanted to increase the AMP size of the bank. While I had a place to install a 2nd 4D, trying to get a 120lb battery into the space was near impossible. Someone suggested and I went with it was to install two 6V wet cell batteries tied in series in place of the 4D. I picked up two Trojan golf cart batteries for $100 each. The total AMP capacity jumped from 180 to 225 and they fit in the same space the 4D did. Additionally they're better able to handle the lower but extended draw we typically see while on the hook. Their minutes reserve at a 20 amp rate is almost 2x a single 4D. Not sure you can make them fit where your 27s are but it may be worth a try. Good luckMike