How many rpms does a 454 like on the highway?

[tucsonjwt]

I have a 1983 C20 camper special with 454, SM465 granny 4 speed, and 4:10 rear axle. When I travel at 55-60 the engine is screaming. I don't know the rpm because I don't have a tach. When my Gear Vendor overdrive was still working it was not so bad. GV says they reduce the final drive ratio by about 20%, which is probably correct in my case. I was getting 11 city and 13 highway with the GV working, but now that it is not working I get about 9 city and 11 highway, but I almost never take it on the highway.

The local Gear Vendor mechanic recommended replacing the granny 4 speed with an NV4500, removing the GV and drive shaft, and installing a new factory length drive shaft, and possibly a highway rear diff (maybe 3.23 or 3.73.) The cost of all that would be about $6-7K, so not practical since I only drive my truck about 800 miles per year to pick up long items from the home centers or go to the dump. Just swapping the rear diff for a 3.23 is probably the most cost effective for me, but that reduces the CGWR significantly, and is not justified based on how little I drive the truck.