I'm going to change my harmonic balancer. I have a vibration at 2500, and also am throwing my fan belt at the end of the 1/8th mile track when I back off the gas. I have a 8" damper now, I see there are other sizes available for 350. How do I know which to use.
rj

Personally I like the Old 68-69 9" or thick 7 1/4 327 Harmonic Balancer but I'm not into Drag Racing. I seems to me the smaller diameter and thinner early 283 or 67 327 Balancer is the one to use if you Drag Race. Of course you will require the corresponding timing cover.

I can't say that any Harmonic Damper will solve your Vibration Problem with out the Rotating Assembly being balanced but it's possible the Torque Converter or FlyWheel/Clutch Assembly is the cause.