Mystery vibration at 25 mph can't be diagnosed. It's not the front end!

Killer6, I believe everything is original to '65, but I gotta sneaking feeling that it's going to end up to be the converter, flex plate, etc.. Lord, I hope not. I'll try everything else first. Pishta, you scared me. I took the dart to the car wash and cleaned the engine. All the grime was hiding the mounting fasteners for the steering box. Cleaned it up and tightened bolts and inspected the K member. I didn't see any cracks, but if I do find cracks, can it be welded for structural integrity?