Carl: before i go replacing anything i would check and see if the weighted balancer didn't come off the drive shaft...Push the drive shaft all the way into the tranny and TRACE a black majic marker line on the drive shaft yoke at the end of the tranny housing...then pull the drivshaft out until it goes into the coupler on the rear end...then look back at the yoke where you TRACED THE LINE this will show you how much drive shaft you have going up into the tranny.....this will tell you if your too long, too short etc...
Please be aware that the drive shaft can throw off a balancer weight in a heart beat..particualry on a factory type drive shaft....
but i would start there and see if this doesn't clear it up..
i see your Chevelle well into the 12's once you can stay into it...
Brian
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "