Drivetrain Vibration

I had a similar issue with my 70 Swinger. It turned out to be a clutch pressure plate that threw a weight, or wasn't balanced properly. Id have the torque converter checked, see if it has counter weights or if it looks like it lost a weight. Stuff like this is frustrating, but I got real good at having that 4 speed out in less than an hour, with a lift and air tools of course ;)