I have since sold that 2.5" truck manifold.
To tune the 390 carb, the running mixture at cruise can be leaned up by changing the main jets to a smaller size. Many time after doing that the WOT mixture will be lean. This is corrected by opening up the PVCR holes with micro sized drill bits. Figure out the size of existing PVCR and then open it to the nize bit in the set. Do not just drill away with any bigger bit! This is something that is hard to reverse after doing.
What this does is lean up the mixture with jets but yet supply extra fuel thru the PVCR circuit to make up for the smaller jets.
