Progressive two-barrels were designed for high RPM (6000 or more) four cylinder engines. The small primary allows higher air velocity at low RPM for good efficiency; and the secondary allows for more CFM at higher RPM's.

Under no condition will a two-barrel of any kind work better (or even as well) on a one-barrel manifold than the one-barrel of maximum size for the manifold (the hole in the manifold is the limiting factor).

So two important considerations:

(1) Do you plan on 6000 plus RPM with your 261 (progressive 2-barrel)
(2) Do you have a two-barrel manifold for your 261 (any 2-barrel)

Your comment about using a Zenith 1-barrel from a GMC 270 would be a better way to go.

Jon.

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