Best bet would be to consider an overdrive. A 3 speed overdrive from a '55-'57 Passenger car or '55-'59 Truck is a direct bolt in. You should be able to find a good used one in the $100-$200 range. I've ran mine (235, od trans, 3.90 gears, 30" tires) at 75+ mph, no problems at all.
