I had the same dilemma. I went with optima in the 69 race car mounted in the right box, because racecar! no spare tire... The optima can work in the left box for street if you do not have a charcoal canister. made an extension strap to maintain the factory latch holdown in either case.
However, I had to install a new battery in a 72 street car, full size spare tire and charcoal canister. Went with the odyessyPC925 sealed AGM with sae posts from sepbatteries dot com at $153 and free shipping. I had a pair of factory dead lead acid batteries, side by side, I cut a 2 x 10 to match the height and width, notched around the posts and no modification to the factory latching and no movement or worry over potential shorting.
