After 50 years, it's entirely possible that something could be bent - like the driveshaft or an axle. I'd check the wear parts first, then the hard parts if the problem is still there. My old daily driver Talon had a driveline shimmy that I couldn't get rid of for anything, so I feel your pain.
BTW, 3500 RPM sustained sounds like a lot for a 235. There's a reason why the old '50s drag racers dropped GMC engines into Chevy cars...
