Assuming you've got an original-equipment engine and drivetrain, I'd suggest a max cruise RPM of around 3000, assuming the lower end is in very good condition and the oil pressure is good. Be aware that the engine will be a little noisy, mostly from the fan being wound up pretty tight. The 54 235 engine should have aluminum pistons and insert-type rod bearings, so pounding Babbit bearings out with cast iron boat anchors shouldn't be a problem. Short runs above 3K shouldn't do much damage, but most folks just can't bring themselves to run a stovebolt that fast.
Back in my college days, I ran a 58 Chevy Delray coupe with a 235 regularly at 70+ MPH with a 3 speed and a 3.70 rear end, and 14" wheels. Yes, it sounded a little stressed, but it stayed together for a long time.
Jerry
