Hi Again,
I've now double checked to make sure it's the right balancer and the flexplate is also correct and straight. But my bottom pulley is slightly bent. However: I've traced the real problem to a completely different source: The ignition. I just thought I might try and fiddle with it, and hey presto, it did it! The vibrtion isn't completey gone, I can still feel it at highway speeds, but nowhere near as bad as it used to be. Seems to be my vac-canister, which is very sensitive to even small differences in vacuum. I always set my ignition the same, 8° BTDC static and left the rest to the dizzy. But somehow that didn't do it. As soon as I have time I'll tune up my ignition properly.
I still have that vibration issue, but I don't think it will cause any further problems, it's just very slight. Somehow my bad ignition seems to have amplified the problem...
Thanks for all the help, I'll keep you posted as soon as I find the actual source of the vibration.
CU,
Mad