57ExtCab,
If you have the Corporate GM 15,000# 2-speed rear with vacuum shift (which I bet you do), than the "standard" rear ratio used in the TF trucks was 8.72/6.40:1. The parts book has a faster set of gears listed that are 8.39/6.17:1, but I've never seen this gear ratio...don't know what they came in.
How fast are you look'n to go? Any thing faster than about 65-70 and your out running you brakes if they are stock.
Installing a OD trans or by adding a OD unit like Gear Vendors or US Gear make might be easier to do.
Mike B
