Pinion Angle/Tailshaft Bushing

My '68 suffers from a vibration that begins around 100 mph and gets worse up through 140mph. It's bad enough that I have not exceeded that speed. It's silky smooth at all speeds up to that. Interestingly, if I coast at 100mph and let it slow down to 95mph, it goes away. It also goes away at 100mph to 95 mph, if I put light pressure on the throttle. That indicates it wants a bit less downward pinion angle. I do have slapper bars with the snubber slightly touching the front spring eye to limit the rotation.

I used the special tool to replace the tailshaft bushing in my transmission with it in the car. It took about half an hour with the tool to remove and replace the bushing. It cost about $150 US. Tire balance has been checked several times too.

My trans is at 0 degrees, the pinion is down 2 degrees in relation to that. Driveshaft has been checked for balance and my local driveline guy says it is dead on. The input yoke is a forged Mark Williams unit. I believe the clearance is still excessive between the yoke and the tailshaft bushing as I can move it with a bit of effort.

If I could get an accurate measurement of the inside diameter of the installed tailshaft bushing, there is a local shop nearby that repairs aircraft and production machinery. They could hard chrome plate and build up and surface of the yoke and then finish grind the yoke surface to any larger diameter I spec. Does anyone know what the ideal clearance is?

Once of the problems I have is figuring out how to accuratly measure the bushing with the output shaft of the transmission in the way.