Sounds slow. A 235 can spin 3200 rpm all day long. 3200 rpm with 3.9 gears and 28 inch tall tires is 68 mph.
I believe HydraMatic equipped 1/2 ton trucks came with a 3.38:1 ratio. That would give 79 mph as a top speed.
Find out what ratio you have first. 3.73 gears may end up being a step backwards for you.
