MuuMuu101's 68 Dart, A Learning Process...

I wouldn't get too discouraged - it could be as simple as a loose bolt or two under the car for the vibration. Additionally, the throwout bearing may not be adjusted correctly and not have enough freeplay - though you would think a pro shop would have gotten that right, but I've been surprised before.

Best course of action is to look over the bottom of the car for missing bolts first and then go from there. I don't know if you have much to look through at the clutch on your setup, but I'd make sure the throw out bearing has some clearance to the clutch fingers.

Modified cars and old cars have teething problems - sometimes it takes a bit to get it all worked out.