Most 6volt golf cart batteries are the same dimensions for the same amp hour rating . Not sure why new ones would be taller. True 12 volt deep cycle batteries are larger ( wider and longer) and hard to find . 4 won't fit in the same space as 4 golf carts. And no you won't maintain the same amp hours . Are you wanting to replace 4 deep cycle 6 with smaller starter or "marine" 12 volt batteries ? As already said if you don't ever "boondock or overnight without power then even two 12 volt marine batteries . might be just fine. Giving power for a few hours.