The high-compression big-valve non-RA D-port heads flow essentially the same as the D-port RA heads. In many cases both RA and non-RA used the same exact head.
All of the high compression 400 and 428 head castings (12, 13, 16, 31, 48, 62, 670) flow pretty much the same, there is no real 'winner' in the bunch. There is a slight variation in combustion chamber size among them, ranging from about 70 to 80 ccs. Take your pick, run whichever one you have available.
Even the later low-compression heads with the 2.11 intake valve are pretty much equal to one another, but some will respond better to porting than others when it comes to full-race porting jobs (6X being one of the favorites).
How much ET can you gain from a port job? Impossible to answer since every engine/car/tranny/rear gear combo is different.