engine problems

I put together a 496 rat and installed it in a '75 Blazer. It only has two problems and I've done everything I can think of to fix them. From my observations off the balancer, it will run perfectly smooth and then it
advances a quarter to 3/8 inch, and retards the same distance, and then it returns to a smooth idle. Also, my light gun is measuring it at around 70 degrees advance. I know it shouldn't be that high. In face, that's about twice as much as it should be. I'm wondering, since the balancer is the only thing that isn't new on the engine, besides the block itself, could the balancer be worn out to the point that it's slipping on the rubber buffer and that's what's responsible for the bad readings and the resulting resonance is causing the rough idle?
The engine is a 10:1 with a 750 Edelbrock Performer carb, and performer intake. There are no vaccuum leaks that I have detected, and the fuel mixture is set correctly. I'm running ACDelco R45TS and have switched out to R43TS but the problem remains. The fuel is 91 octane with a bottle of 104 octane booster mixed in the suggested ratio.
The only other thing I can think it might be is maybe I put too much oil in the cylinders during construction and just need to get it out on the road to burn it out.
Any suggestions?
JC
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and installed on an engine correctly, You don't just pry it off and hammer it back on. It is very possiable that the balancer is messed up.
-- Diamond Jim The Old Devildog
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