The easy button (for me) to get a decent cruise speed with a stock 85 HP engine (net) is a S10 overdrive transmission. I know your plan is to keep the original transmission. That said, unless you expect to work the truck like a farmer/rancher would have 70 years ago, you give up nothing with the modern S10. Well that is unless a easy shifting, highway happy transmission is a hardship.
Lots of reading via search on the subject.
Good Luck,
RonR
