Well you didn't say what size carb your trying to use on the PES intake.But no dout it's to small.Secondly.Even if the clifford runners did measure the same size, There is a VERY Big difference
in the ruuners performance.Due to the over all design. When it came to intakes My 3x4 tunnel ram out performed The offey and the Clifford.Reasoning I beleive is the 90 deg. runner design.
The PES intake will need More carb.Or at the very least bigger jetting on the smaller carb.