UGE dampener. Is that the same as the quad shock on the Mustang?I had the same problem after going from 302 to a 347 with 28oz flywheel. It vibrated at 60mph in 3rd gear - and 3rd gear only. Pinion angle was OK. I had a 4-link and I bolted back on that UGE dampener from the Mustang. That solved the problem. (I later went to a Levy Racing 5-link)
You said you had 4-link, curious - are you using the dampener?