Did it feel like tire shake due to imbalance or was it more of a wheel-hop? If you're running a rim like the Weld Draglite, its tough to balance them on an ordinary spin balancer. The Welds need to be torqued so that the three pieces draw together tight. Weld recommends balancing them while mounted on the car if you can find anybody around that still does that. I run mine unbalanced and really don't notice it much until the big end of the track. If it was wheel-hop that you were experiencing, then you need to work on the suspension. With a '69 and its leaf springs, you're going to need some sort of traction/slapper bar to overcome the wheel-hop. You'll have to adjust the slapper clearance as well as pinion angle to get the most out of them. Another possibility would be driveshaft imbalance.
------------------
Jeff Chambers
T/S #4
12.07 Seconds/112.6 MPH
http://www.kellnet.com/chambers/default.htm