I have a progressive Harper, the outboard side opens normally and the inboard side starts to open at part throttle, they reach WOT at the same time. I beleve the GMC and Chev manifolds were the same as they have both attachment features, just change the length of the idle balance tubes. I will try to get some pictures, and will bring it to the 2012 convention, if I still have it.

Tim

Tim Tenold
I.I.#498