The 390 is ideal (assuming the manifold is good) with those tiny primaries.
Just for a test: only change the size of 1 main jet at a time (yes, it may do a bad mixture distribution thing), the point is to approach best cruising vacuum in small steps.
To get the mpg really big, pick a PV that remains closed at any speed you will reach, even with mild acceleration. Accept the slightly slow tip-in response as the price of better mileage.
