454/dana 70...highway gears?

[Puglia10]

I have a 1979 GMC crew cab dually. 454, th400, and a dana70HD rear end.
I still need to figure out what gears I have in there, but I assume 4.10 or 4.56 because the motor is screaming at 60mph.

I'd like it to be more highway friendly (70 all day, 75 would be nice) I'm not terribly concerned with gas mileage because it's a toy, and will only be towing a couple Harleys, or one hot rod...nothing crazy heavy, and even that will be rare. Mostly just building a fun cruiser that I can take to some shows...and again, maybe bring a couple bikes. Pulling a car MIGHT be once a year.

I know gear vendors or 4l80e would probably make the most sense, but for a quick and inexpensive fix, I am wondering what gear ratio would make sense for the highway driving, but still be able to move the big truck (with 22" Alcoas and adapters) and go up a hill.

At what point is the gearing too high that it won't be able to move around town?
Thanks!