I have a '45 GMC that has Patrick's 3.55 R&P in it. Unless the price went up dramatically, You should expect to pay $399.00 for the R&P and another $275 for an install kit with new bearings etc et etc.
http://www.patricksantiquecars.com/chevyparts.htm Then there's labor for a competent mechanic if you're not up to the challenge.
If you put a new axle in, then you need an open driveline, new transmission, new pedal setup, new master cyl, setup the speedo, hack the floor. and on and on. Save yourself the bother Tony and sell the truck to me.
