Everybody, and I mean EVERYBODY that builds old cars/trucks needs to keep this site on hand.
http://www.700r4.com Just measure the heigth of your rear tire and know your rear end ratio and go to "calculate speedometer" on this site and it gives the exact gears you need--the drive and driven gears. The drive gear on the tailshaft requires taking off the tailhousing but that not a hard job, and it works every time.
