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Gregsdart runs the 440-1 heads an make somewhere around 900hp, so I would imagine your combination isn't maxed out. Generally speaking, adding more cubic inches will bring down your peak power RPM as you stated. If all else remains the same, I would imagine the gains would be minimal.
Here's a  for you 
You could send them out to have them ported to the max, if the 325 has enough meat left in them. Do they have the 2.25 intake valves? That would be my first move. Consider buying the valves a touch longer for better installed height if you are going up in valve size. It is tough (maybe impossible)to get more than 2.050 out of the standard length valves. I have plus retainers and keepers to get there.
I started with a max ported 440-1 and told the porter that I wanted lots of runner volume for Alky, and they run pretty good for a 528 cube motor. I also run the 3X intake.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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