I am not the best person to advise anyone just ask around but in my 54 1/2 ton, I have a 1979 Nova rear end with 2.73 gears and an s10 5 speed. But I live in Indiana and it is usually flat and it works for me. I have been down through the Smokeys and on the interstate, it does fine but on back roads, it takes a little shifting. Best of luck.

With a 3 on the tree, I would be looking for a 3.73 out of a '67-'70 half ton truck. Its a 12 bolt rear end. You will have to weld on some spring perches.
Any ratio numerically less in number (like say, a 3.55) would be a sluggish in first gear under any kind of an incline.
Carl

Another option would be to swap the transmission for an overdrive unit instead of swapping the rear axle. The SM319 was an option on these trucks from the factory, so they are a direct bolt in, even using one from a Tri-5 passenger car. I have one in mine, and 70+ is no problem for the 235.

When the question of top speed comes up, I always ask myself about braking.

I had a serious case of drum-brake fade in our '65 Chev on I-80 in Wyoming one time (~1969) when a big buck deer, trying to block most of the 2 lanes, stared me down. I couldn't stop but thankfully I could steer a bit. Drifted right by him.

45 MPH is great for me.

I’m not familiar with the sm 319. Does it shift the same as a 3 speed? How do you shift it to overdrive? Do you know if a driveshaft mod is required? Thank you for the help!

I thought the same on my first drive in my 37,that problem has been solved with a T-5 and 3.55 gears.

The SM 319 is a variant of the SM 318 3 speed that is standard in the '55-'66 trucks. Both transmissions also saw use in GM passenger cars in the same time period. The SM 319 is setup with a Borg-Warner overdrive unit in place of the tailshaft and its housing.
The main part of the transmission is the same as the standard 3 speed, so the shifter hookup and operation is identical. The overdrive is, if installed as the factory did it, shifted electrically via a solenoid on the housing. The shifting is done automatically by lifting off the throttle at approximately 28 mph (that speed can vary depending on rear ratio and tire size), which is when the contacts in the governor close. Overdrive can be used in any forward gear. There is also a manual lockout cable which locks the overdrive, making the transmission operate as a normal 3 speed.
If you use a transmission from a '55/56/57 Chevrolet Passenger car or '55-'59 Truck, no driveshaft mods will be required. The only change, depending on what the trans came from, would be to swap the shift cover out as Passenger and '55 6 cylinder Truck versions are different from '55-'59 Truck.
Here's the shop manual section from the '55 manual can explain things a little better on operation http://chevy.oldcarmanualproject.com/shop/1955truck2/55ctsm0715.htm
I've had one in mine for the last 20 years, it's probably the best thing I did to it.

40 - 45 mph in my truck with standard (1956) 3.9 rear is about 1800 rpm . A standard 235 1/2 ton should cruise easy at 60mph , it will go faster , but your mileage will go away

If you have too much noise , determine where it is coming from and fix that . I predict it may be a cheaper option .

Jerrys "pull the fan off" is a good way to isolate noise issues.

Consider taller tyres as a small gearing adjustment .

I agree with the brake comments .

Great info guys I really appreciate it. Love the idea of the sm319. I’ll look into finding one. I did the brakes with a hidden booster and discs on the front so she stops great. Not that I want to go that fast, I just want to go with the flow of traffic.

Took my truck out for a spin yesterday. With 3.55 gears in back behind a SM420, my RPM's are 2200 at 55 MPH, 2600 at 60 MPH.