I've owned a lot of cruisers, with F's, 2F's, 3B's, 2H, 283chev, 350 chev.
So I feel I can do a fair comparison on abilities of certain drivetrains
There are a few reasons why I don't travel slow
My SM420 is a four speed, first is so damn low it is useless on the street
So call it a three speed
top gear (4th) is one to one and combined with the V8 gets you around quite nicely in fourth
Most hills up or down are done in fourth.
Whipsaw is a good example. Most club members are familiar with this trip. Coming home from the trailhead in long weekend traffic, I need to use third going up most hills due to the volume of slow traffic. If there is no traffic, I only use third on a few sharp up hill corners due to the fact I lean so much. I fear seeing asphalt that close to my door handles ever since Bruce in his weeble wagon showed me he could pull off 45+ degree leans while going in a straight line if he wiggled the steering wheel.
so it is pretty simple
fourth gear on hills = smooth engine, better mileage, quieter cab noise, fresh air, extra time to stop at gas stations to fill up.
thirds gear on hills = higher RPM, way worse mileage, noisy as a room full of screaming kids, smell of diesel and KFC in the air, and having to make diesels pull over while I have to fill up more then twice as often.
I've had thoughts of a 13BT or 12HT going into one of my FJ45's. That will be a future project as well because both of the FJ45's are low mileage cruisers that run just fine on their stock F and 2F.
If it ain't broke don't fix it!