exhaust theads stripped

Hi, frist time on this form,thank god I found it!.I have 74 dart sport 318 4spd
and had to change the starter, I removed all bolts from the header and changed the starter,went to reinstall the header and the last bolt(the one that is part of the cooling passages) back of the block is stripped out.what would be the best thing to do? Can I retap it to 3/8ths or use a heily coil .I am open to any suggestions .Thanks