Depending on the dia. of the hole,you may be able to use a roll tap.A roll tap does not cut but forms the thread so there will be no chips.

 

I stripped one also installing new crome lowers a couple years ago, just take the screw think its a 5/16 18tpi, to the the HW store and buy the next lenght buttion head , there will still be a few threads left in the leg ,, make sure the new screw is not too long so it hits the internals, use a new crush washer wrap the longer screw with teflon tape and don't over tighten it,the teflon will do most of the sealing, worked for me, my next step if it didn't work was to tap to 1/8 NPT and use a SS hex pipe plug,,, good luck