Off the top of my head...
Looks like OEM replicas are listed as 6.25"... about an inch wider in diameter than some of the aftermarket pulleys I found.
Not sure what diameter the Moroso is, only that it says a 30% reduction in pulley speed, IIRC. My brain hasn't shifted into calculator mode to reverse-engineer the diameter.
To figure what i needed i took the max alternater rpm. As my target, then from the alternater pulley diameter and my max engine rpm i came up with the size pulley i needed. Since most all of us run more rpm at cruise than a stock setup it works well for the water pump also. Mymax rpm on the 408 was 6200 and a 5 inch pulley was about perfect. I had 3.55 gears and an auto.
