Need some guidance tracking down a vibration

[dvw]

The pulley is not the issue. One hole is offset in the balancer. You can oblong one hole in the pulley if it bothers you. This leaves two other possibility's. #1, Something is grounded out. Exhaust,trans, hitting the chassis. Motor or trans mount sagged, bound up or broken. #2 Rotating Assembly out of balance. If it's a factory short block the most likely cause would be 6 Pak rods with a standard balancer. A 6 pak balancer has an off set weight similar to a cast crank but smaller. Most 70 and 4 barrel motors were built with 6 Pak rods. If the short block was built with after market parts it may need to be balanced. Most out of balance rotating assembly's I've seen were the most prevalent around 3000-3500 rpm.
Doug

 

[zimmstoy]

Update: Had a mechanic friend come over today. Looks like the issue may be exhaust related. There is some shoddy work underneath the car that needs to be corrected. Does anyone know what the stock pipe size was that came with these cars? Was is 2.25"?

 

[RustyRatRod]

Well good. Maybe it's somethin as simple as a grounded exhaust pipe.