Hi pft and welcome!
I had the rear end of my 1956 chevy 1/2 ton changed from 3.9 to 3.38. Having it done professionally (drop off in the morning, pick up in the afternoon) cost me $1200, including new seals. I think $500-700 if you do it yourself with new parts. You will want to upgrade your brakes and switch to tapered roller bearings for front axle, particularly for a DD. Lots of info on the forums for each of these changes.
It keeps up with freeway traffic pretty easily now but has noticeably less torque climbing hills. If I had to do it over, I would have spent the money on an overdrive transmission. T-5 is very popular upgrade, especially behind a 235. Check the Tech Tips for information.
