I would go with the same size tires all around. It will save you a lot of money being able to rotate your tires. I would also get that leak fixed first, it's likely your heater core and the repair can be expensive if you have a shop do it. My sister's 83 mustang had a heater core leak and we were quoted over $700 not including parts to get it fixed. The dash has to be partially removed and that is very time consuming. Some shops have developed special techniques and do it for around $100 including parts now so don't forget to look around.