Vibration Problem

Is this a 4 speed car? The reason I ask is that I had the same kind of problem that you have. I checked everything up front on the motor and all was fine. But I ended up finding was my Centerforce pressure plate had 2 ball bearing work loose in the diaphram arms to throw things out of balance. I had to take the pressure plate completely loose, that's when the ball bearings fell out on the garage floor! It was a new clutch too! This may not be the case with your car My situation happened from one day to the next and got noticably worse over the next couple of days! One thing to try to eliminate it if it is up front is to disconnect the v belt or belts and start the motor and run it at those rpms where the vib comes in and see if it goes away. If you have multiple belts, you'll have to do one at a time to be sure of where it's coming from. That's how I knew to look to the rear of my 340 because nothing up front made any difference when connected or not.

Good Luck!