Quick answer: no more than 2400 rpm.
Longer answer (including the "why?" answer to the statement above): The torque/horsepower curves were different for different years of the 235 and they're interesting and well worth studying. Your sweet spot (1954) is going to be somewhere around 2000 to 2400 rpm. At 2400 rpm your torque line begins a steep incline downward and intersects the horsepower line around 2550~2600 rpm. Beyond this rpm point, efficiency simply won't be what you might wish it was. So I'd say keep your cruising speed at 50 mph if you have the stock rear (4.11) and 28 inch diameter tires (that will be apx 2400 rpm). And this will match what driving one of these trucks (in stock condition) for over 50 years has proven to me time and time and time again. Hills and wind will be a problem for gas mileage, naturally. But if gas mileage isn't a concern, then drive at whatever speed you consider safe. There is (or used to be) one GM internal memo on the Heritage Page site showing the AD truck was designed for a safe highway speed of 50 to 55 mph. I had it bookmarked, however GM changed the Heritage Pages and yesterday's bookmarks don't all work any longer plus some data disappeared. I wish I had downloaded and saved it, but I didn't
Here is a link to the Heritage Pages for your truck:
https://www.gm.com/content/dam/comp...hevrolet-trucks/1954-Chevrolet-Truck.pdfAnd if you're curious to see what the torque curve for the 1959 235 looks like (it is much different---is a long flat line instead of a peaked one), here is a link to that data:
https://www.gm.com/content/dam/comp...hevrolet-trucks/1959-Chevrolet-Truck.pdfGM managed to get this long flat line by altering the camshaft (that data and the explanation was once also on the GM site, but I haven't been able to find it now). This was done to try to help reduce smog as I understand...produce more power at a lower rpm range and keep that power band consistent through several hundred rpm. The 1959 235 hits close to max torque at 1600 rpm and doesn't lose it until around 2800rpm. Study the facts regarding the camshafts used and you can see the differences.
Good luck on your trip!
