[kwmech]

You'll have to change the posi when you go to a 'taller' gear set. 3.42 ratio will work just fine with the tire size. You could even go with a 3.08. The big block will have plenty of power to pull through the gears. And please don't waste your time and money with a 700....... I don't care what anybody says about building them tougher to last behind a big block. Simply stated they don't last. If you want to go the replacing the transmission route, put in a 4L80E and never worry about it again. Another bullet proof solution without a controller is the Gear Vendor (as previously mentioned), it will bolt right on the back of your 400, a little bit of wiring and shortening of your driveshaft and you're done. They are a tough unit and will last quite a while also.