Because the Performer is a 4 bbl manifold. If you want to run a 2 bbl get a 2 bbl manifold.Please explain why it is that you think square vs spread bore has anything to do with that.
Edelbrock did at one time sell some manifolds in this configuration, they may still, and they have/had EO numbers.
QuadraJet is a very efficient carburator vs the Edelbrock, which is based on the Carter AFB. GM no longer used them after 1966 as far as I know; GM used Q-Jets. In late '66 the Q-Jet started to be used.