I would ditch the pinion snubber, run a long rear shock, put the 650 on ,advance timing to 36-38 total (use good gas)and have someone verify you don't spin! use shoe polish and mark the tire with a line. Also verify your timing! Balancer could be off. Just for giggles put a vacuum gauge on it and advance timing till it peaks on gauge,then back down the timing 1 " of vacuum on the gauge,then see how it runs.