I am using a 1949 SM420 in my '51 burb. The speedo was way off when I put it in, but I took the truck to a local old time speedo shop, and they fixed me up with a very clever little gear box which attaches to the tranny and puts out the right ratio. They did this by putting the truck on a roller/treadmill machine and ran it at some speed like 50 or 60 miles and hour, did a little math and then assembled the little gear box using gears from a big chest.
They have since gone the way of the dinosaurs, but maybe you can find somebody to do that for you.
Kurt
