Mystery vibration at 25 mph can't be diagnosed. It's not the front end!

B&T is 2 rollers on a shaft and a receiver. If you cant shake the driveshaft back and forth/up and down along the plane of the balls or rotationally at the trans output shaft, the B&T is probably not at fault, these were installed behind street hemis in heavy cars so they can take some torque. IF the B&T has worn itself a groove due to lack of lube from a disintegrating 50 year old rubber/leather sack (likely) the washers will move the receiver a few mm down its length to virgin territory. A 4X4 shop can make you a splined driveshaft that will bolt up to your old flange and rear end. MP used to sell the kit for this so its a legit conversion. IF your mechanic sensed a vibe at speed with the driveshaft removed (?) then its not the reason for THAT issue logically. I would start to investigate myself with little effort here: remove the belt(s) and rev the motor to replicate that vibration. Could be nothing but a bent fan or water pump bearing. The ignition upgrade sounds like it went great. IS this a manual trans? IF it were an automatic, Id tell you to check the convertor bolts and the flex plate. all convertors have balance weights on them. Id look while I was down there for any unpainted areas on the convertor perimeter for traces of a weight that was flung off. The timing chain is unlikely going to cause a ignition issue dramatic enough to cause a vibration. These slants are pretty low tech when it comes to tolerances in most all areas, 'cept maybe valve lash. They were made tough for all kinds of applications. I had my first high school 65 up that high for tire reasons (they were too wide) and I think I always had a nagging shudder at some speed, most likely due to the angle of the pinion compared to the angle of the trans output shaft. There is a tolerable range for any double U-joint (its rotation/speed variance can be likened to a sineusoidal wavelength) but Im not 100% sure it applies to the CV of the B&T. Hey, you know you can only do so much with a nackered back, so take our suggestions to heart if you cant physically do the work yourself. You know, Id trust a 50 year old mechanic too, unless he was admitedly clueless to that design. Those guys see far more trouble than we do on a daily basis (unless one of us is a mechanic too).