Is the transmission shifting into Fourth gear? A simple adjustment may be needed to ensure all 4 gears are being used.
Depending on gears and tire size, 65 mph should be around 3000 rpm. Using an online calculator I punched in 3.9 rear gears and 28 inch tall tires and came up with 3042rpm. if your truck had 3.38 gears 65 mph would be 2636rpm. put some 31 inch tall tires and you are down to 2381 rpm
There are many rear ends and tire combinations you can use to drop the rpm further. 1957-1959 gmc 1/2 tons had a 3.08 spicer rearend available. This rear would be a near drop in replacement, however brake parts for the spicer rear tend to be challenging to find. Chevy 10 and 12 bolt rears can also be made to work with a little help from an electric glue gun. The 10 and 12 bolts have gear sets available from 2.73:1 through 4.11:1.
I have an IMCA modified 9 inch in my 59 gmc with 3.70:1 posi. I also have an OD transmission which makes the final drive about 2.70:1. I have a big V8 and needed more rearend because the rebuilt eaton HO33 rear with 3.38 gears was literally ripped apart by my new engine. You shouldn't need to go crazy with a 235 6 cylinder.
