51 Chevy coupe question

Ok so I want to put the widest rear tire I can on my 51 coupe without having to change the frame it will have a 3 inch drop any one happen to know off the top what the widest I can fit is?

 

I've got a 15x7 on mine and I can't fit my skirts on.... are you running skirts or not?

 

Most likely no skirts I'm going with the early hot rod look not the drop to the dirt custom