Slick size

You want to run the tallest tire you can fit, then the widest for that height. Taller tires give a longer contact patch than a shorter tire of the same widht. But, knowing 66s a little, I'd think you're stuck with a max diameter of 26 if you're lucky. The wheelwells are just shaped wrong for really big tires. Then you gear and convertor it to get the engine in it's power band and keep it there. With a 25 or 26" tire and 5500 rpm, you are looking at 3.91s or 3.73s...