Need help w/gear and slick size on a 402 Duster

I'm for the largest diameter you can fit in it. I believe without trimming the wheel well that's 28". trap speeds areound 110 you're looking at keeping the rpms down near 5500 at the stripe. So I'd go 3.91s. If it were me, I'd plan to replace teh convertor with a 9.5" unit that stalls around 3400 and test your shift points. I think you may find that it wants to be shifted lower than 5500. At least until the carb is updated as it will be hitting an wall around 5500. Go with something at least 850 dry flowed CFM.