Duster Driveshaft length causing Vibration?

I just measured the drive shaft from my 71 demon 340 car with 727 and 8 3/4 diff. I used this shaft in my 74 duster with a big block & 727 & 8 3/4. It was a perfect fit. 45 inches ctc of the u joints. Never any vibration. I agree your drive shaft is 1 inch too short. Lower the pinion at least 2 degrees more than the trans angle. Kim