Need suggestions on vibration issue

Make sure your flywheel is 360 balance one and your dampener is 360 as well, coarse if it's been driving that long and aren't the proper balance parts for the 360 it would most likely have shaken it's self to death already.

All the engines I build for 4spd cars are balanced at the machine shop (it can still be externally balanced in the case of a 360 but the rotating assembly has to be balanced in order for it to be smooth) it makes a huge difference in how the engine runs.

I am guessing it was an automatic engine that they put in and isn't finish balanced like it should be.