Love the project
I've always thought a big V8 can still hit great number with EOC, and I just picked up my own V8 project truck.
Regarding hypermiling and towing, I have the same issue, I do a lot of towing and towing in city traffic, so the ratio choice is always a big compromise. So, I've been thinking about sourcing a two speed (external overdrive) diff like was popular with British cars in the 60's-70's. Most often these could only be engaged in 3rd and 4th, but I don't see why that couldn't be modded out. Two speed diffs these days are still around in some smaller trucks like the Iveco Daily, which might be too big for your use, and the ratios might not suit, but it would be a lot easier than swapping diffs every time you want to tow something.
The other thing I'm going to to is build a much lighter trailer