Engine/drivetrain vibration?

Been here, done this.............here we go again................

I feel your pain. If I remember correctly, the 340's are internally balanced. This means the balancer AND the converter must have NO counterweights on them....BUT, different years could be external......not sure.

Get under the car, and check the back side of the balancer as someone bumps your key to make sure you look at the entire balancer........If there's a counterweight sunken into the back, take it off if your 340 checks out to be internal balance...........probably held in by an allen set screw.

If you have no counterweights, then something else is probably out of balance somewhere........could be a yoke, the driveshaft, tailshaft, etc., OR in the motor. If your boy didn't balance the parts before he installed, it could be your culprit.

Some other guys here have great experience with this type of thing too...........Let's see if they chime in. Good luck to ya'......and be sure to post your findings!

Bottom line is, I believe, this: All of the small blocks were internally balanced except for the 360, and cast-crank 340.....yours is probably a cast crank if it's original, and should need counterweights...but you'd better be sure it's the original for 73.