Hello I have been chasing down a vibration on my 1975 valiant. Here is some back ground information. 360 block turned into a 408 stroker with a 727 three speed. The vibration starts around 50 MPH and 2500 RPM. I have balanced the tires and drive shaft twice. Installed new u- joints, 3000 B/M hole shot converter and B/M flex plate. Verified that the block is correctly balanced. Removed the tires and brake drums and returned the car to the correct ride height. Brought the speed up to 50 and the vibration was still there. Today I went for a drive keeping the tranny in 2nd. I brought the speed up to 50 then 55 no vibration. Maintaining 55 I shifted into 3rd and the vibration immediately appeared. I repeated this test several time with the same results. I also tested out the RPM. In 3rd at 2500 rpm I had a vibration and in second at 2500 rpm there was no vibration. I'm thinking it something to do with 3rd gear in the tranny, but i have spent hours looking online and can find nothing.