If memory serves me correctly, the old school battery size is 24 for Skylarks. The heavy duty battery for the '65 GS calls for a 24T (T = tall, I assume). 15 years ago when I was looking for one for my convertible, they were not available. So I put a spacer under the battery to bring it up to the proper height. What I used was a spacer that is used in between batteries when they are stacked on top of each other for shipping or storage. The spacer was made of plastic and protected the battery posts on the bottom stacked battery. I don't know if these are still used, but might be worth a call to your local tire and battery store.
John