I'd get a different balancer if it was me, one from a later engine (1960's-mid70's). The one you have is one of the worst as far as dampening ability, because it is so small. You have an ancient relic from the early 265 or 283 cube engines, as the TechInspector guessed. All later balancers on larger and more powerful small blocks were a lot thicker in the outside weight, which makes them more effective at canceling vibrations, where yours is just a thin ring. It was only used on early and low performance applications.
You will have a hard time getting a pulley to run true, unless you can find the early (1950's era)CAST IRON pulley that goes with that balancer.
The later pulleys and dampeners have a centering lip on the pulley the fits just into the crankshaft opening of the balancer snugly.