Everywhere I've read has put .060 as the safest maximum overbore for a normal, standard built street engine. If you can
find them, the LPG application pistons are ideal, for the money. They'll give you a 9:1 compression ratio right off the bat, and with very little extra work you can get 10:1 with them. They'll show up on Ebay from time to time.
Check with Northern Auto Parts. I bought two sets off them about a year and a half ago (sold one). They frequently sell on Ebay (as "mpcentral"). Good people to deal with.
Of course another option is to have them custom made. That won't be cheap (probably $100-$125 a hole - i.e. $600-$625)!
Then again you can get 10:1 with the stock dished piston and milling or shaving the head. If you plan to run a cam that puts your max power and torque higher up in the powerband, then you might want to go the custom made piston route. You can then have them made significantly lighter than the stock ones, making higher rpms much easier on the rest of the rotating assembly.
-magic mike-