Don't worry about topping off the cider. There should be enough CO2 coming out of the cider when you rack it to avoid contamination. Plus, Oxidation isn't much of a problem with ciders. Since you have so many peaches, start another batch.
You might consider adding three or four pounds of very ripe peaches and some Campden tablets. The Campden will kill the yeast & this will give you a little extra sweetness.
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