Hi Mr. Dane,
There are folks who say you can drive the 235 at 3000 rpm all day long and that's totally true if your engine is in good nick. 3000 rpm all day may not make you feel that good, because we've become conditioned to think cruising rpm ought to be around 2000 rpm. And I should mention beyond 3000 rpm you are inviting some unavoidable inefficiencies. When I was running the 4.11 rear and 600 x 16 tires and using an accurate tach, 60 mph occurred at a wee bit less than 3000 rpm. When I switched to 15 inch wheels and 27 inch tires, the rpms increased to the same bit over 3000.
In the 60s and 70s the Dallas Public Library had an entire shelf of bound various GM publications including internal correspondence and in one I read it was suggested 55 mph in the AD truck ought to be considered top highway speed. They had been donated by the GM Arlington plant (which opened in the early 50s). This would have been the 1/2 ton truck with 4.11 rear end. As for max speed, use your own judgement. The 235 has a long, loopy crank, only 4 main bearings and spindly piston rods. And as already mentioned, you're dealing with a front end and brakes that didn't contemplate high speed driving.
