Regarding the original question, I think a stock 170/Ford-O-Matic (as you've found out) is about the bare minimum for freeway travel these days. But unless you're trying to keep your car all stock, there are plenty of parts available to pep up your engine and still keep it reliable. Since an engine is basically an air pump, getting larger volumes of air in & out will give you more horsepower. A slightly larger carb and a freer-flowing exhaust system would help, without having to go inside the engine. A numerically lower differential gear would reduce rpms, but wouldn't help much in the hills.

Even though my Ranchero is a V8 car and I drive it a ton, I try to avoid freeways, as it's no fun just droning along on a freeway. Back roads & small towns are the best way to see America.