How hard to swap 8-3/4" gears?

Hey guys, my friend just got a 1973 Dodge B200 Sportsman van with a 360 (just has 4-bbl and dual exhaust for now, stock heads, cam, etc.). Supposedly the gears in it right now are 4.10:1, and my friend wants to try to put in some taller gears for better gas mileage/highway cruising. I looked at the housing and it's definitely an 8-3/4", and I'm pretty sure is the "489" case (saw those numbers near the pinion). How much would something like a 3.55:1 gearset (and all of the other stuff required for the swap) cost, and how difficult would it be labor-wise to swap the gears?