Might be fine with your current plan, but also consider keeping the 4.56 gears and adding an NV4500 5 speed overdrive transmission.
Grigg
X2
Grigg took the words right out of my mouth. The NV4500 is a great transmission for this application. With the 27% OD the NV4500 offers, your 4.57 rear would feel like a 3.34 rear. The 270 should have no problems pulling this weight on the flat. On a "hilly" road, you will likely have to downshift to direct.
OK, now for some clarifications... You state the truck as a 55.1 with the original 270. The 270 was not offered for 55.1 3/4 ton trucks. The "original" motor should be a 248 gmc. It appears your truck was repowered at some point. Most likely this was done by a previous owner, but it was not uncommon for trucks to get the bigger motor from the warranty service. If the later was the case, there would be a sticker on the firewall that indicates a repower of the vehicle.
