I suppose your intent, like mine was, is to save weight when carrying the batteries into or out of the boat.
Some time ago I replaced the 2 8d's used to start the port 671 with 4d's. That worked fine; however, when it came time to replace them, the owner of the place where I buy batteries suggested a pair of group 27 batteries which were even easier to carry and the two fit in one 8d box leaving the other battery box to store spare engine oil.
All the port batteries do is start the port engine and power the windlass. The 2 smaller batteries seem more than sufficient and the use of a pair of 8d's seems like typical Hatteras over capacity.
That's been my experience.