Buy over-ripe, almost rotten peaches. Rinse them in luke-warm water to get any dirt off.
The acid-producing bacteria on the skins of the fruit will give you a nice, lemony, tartness without the lactic "twang".
DFH produces Festina Lente this way.
PS - i like that you're adding the stone in the secondary. Hopefully that gives you some depth. I'll probably steal that idea on my next peach wheat!