gaffbro -
I've done that on my 37 1/2 ton pick up. I replaced my 4:11 stock rearend with a 54 passenger car POWERGLIDE rear end. It is a 3:54 gear ratio. The 3:73 passenger car STANDARD rear end will help you a little, but not nearly as much as the POWERGLIDE will. There is not that much difference between 4:11 and 3:73. When I did it 20 years ago, I was told NOT to use a STANDARD 3:73 rear end. You'll pick up about ten miles per hour with the 3:54 rear end. It will, obviously, be less with the 3:73 rear end. I don't think five miles per hour is worth the effort.
Length wise, I had to shorten mine. Your wheel base may be about 1 1/2 inches longer than mine, but I think you will have to shorten the torque tube and propeller shaft.
You will go to five lugs on the rear. The U-joint on a 39 truck is 10 splines. I think it went to 17 splines in either 1940 or 1941. The 53 Chevrolet car is 17 splines. To match them up you will have to buy a 53 car u-joint, unless you get it with the rear end and mate the two u-joints together. They will mate together, it's not really a problem, but that is something you will have to do.
To get five lugs on the front of your truck, you will have to have a 41-54 truck axle, which will bolt under your truck, and use the 50-54 car backing plates and drums. They will not fit on the 39 spindle, but will on the 41-54 spindle. The axle is a bolt in changout. Then you'll have five lugs all the way around.
I wrote an article on this and I can email it to you if you drop me your email address. Mine is:
[email protected]
