Hi, Nathan. There is an Inliner named Pat Dykes who has spent some time working on exactly these problems. Your dad's truck probably has a 4.56 or 4.11 rear end. Patrick's, in Casa Grande, AZ, has a 3.55 ring and pinion set designed for that rear end. Pat also makes a conversion kit to put a saginaw 4-speed in your dad's '46 that retains the torque tube driveshaft. A transmission shop should be able to sell you a converter to get the speedo back on track after a gear change.
Patrick's advertizes in Hemming's, and should have a banner ad or link somewhere on this site. Good Luck!
