I did the 3.38 swap in my 56.. here's what I did: I went on ebay and found a good center section with the original 3.90 R&P in it. then I took that and the new 3.38 gear set to an axle/trans shop and said "please rebuild with the new R&P at your leisure". They gave me a good price to do this on a "no rush" basis as 'fill-in' work. I did this for that reason and to avoid disabling the truck. when it was all done I had my new R&P in there, new bearings all around, and the lash properly set (something I was not comfortable doing myself). Then it was a short and simple job to swap out the center sections and be on the road again in a couple of hours. I subsequently sold the 3.90 center section plus the spare R&P to recoup the expense I incurred to get the extra center section.
the swap was well worth doing. my truck, and many of the ones here, do not have to get a heavy load moving any longer and lower revs at roadspeed is a great improvement.
