Unless you're trying to build some sort of 100% original show car, you've been misinformed. Any 6" diameter balancer will fit, up to a mid-1970's 307, and all the mid-1960's 283's. These have a single groove pulley that bolts on with three short 3/8-24 bolts. Brand new 6" balancers are available on Ebay, and so are the single-sheave pulleys. Or, you could search for a 65 year old piece of scrap iron with a riveted pulley- - - -your choice!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!