You shouldn't be having any problem with those numbers. Ideally, the pinion and transmission angles would be parallel, but rear end wrap up would need control. You might add a shim at the transmission mount, but I would look for something else causing the vibration - flywheel imbalance, driveshaft imbalance, bad u-joint, or a rear end bearing or gear problem. Depending on how your 383 was balanced, you could have the wrong flywheel or harmonic balancer. Also, be sure the yoke spline to trans splines are running with enough end to end clearance - with the trans splines not bottoming out in the yoke or the yoke too deep in the trans housing. Sometimes, a simple rotation of 180 degrees of the shaft in the rear yoke will cure vibration.