Mark,
I put the 3:55.1 kit in my '51 last summer. I highly recommend it if you are planning on keeping the truck stock. I just got through putting a fresh 235 in a few weeks ago so now I'm ready for summer cruising.
One thing I have noticed though. With the three speed and the taller rear end it is a little doggier at low speed. I think a four speed would do better. especially if you haul or pull anything.
