340 vibrates
I read your other thread last night...you also said the PO installed a weighted balancer and that was causing the car to vibrate profusely then you installed a "correct" balancer.
Which flexplate did you buy?
Also, I know with internal balance engines, many say you don't need to bring your flexplate and balancer to the machinist for the process, but I figure if I'm paying for it, it's part of the package and I'll let them at least check the balance on those parts...they don't always come out perfect. I know it sounds like I'm beating a dead horse, but you had the shortblock assembled by your machinist and as much as we don't like to believe we'd get hosed, accidents do happen.
OK, enough from me on the imbalance.
Good call 340 Duster...
Possible ignition misfire?
Are all your plug wires good and is the coil firing up to spec? I had a friend with a later model 4.6 T-bird with one side of one coil pack that went back and it shook and ran like hell...it wasn't firing on 2 cylinders because of the "half" bad coil pack. After changing it, it was a whole different animal.