[theastronaut]

My '66 with a 250, 3 speed (same 1:1 top gear ratio as your trans) and 235/75-15 tires will cruise all day at 75 all day no problem, and accelerate up to 85 to pass if needed. I did take the stock fan off and used an HHR electric fan/shroud to free up power, get better mileage, and cut down on engine noise which made quite a noticeable difference, but other than that it's stock. Without overdrive you probably wouldn't want to go much lower than about a 3.42 with a 230 unless your area is flat with no hills.