In order to use pin PB5, which is the reset pin, you need to program it with specific fuse settings. And because you cannot reset it anymore you need a special high voltage programmer. The ICSP method of programming won’t work. And the regular arduino bootloader neither, for sure. Here is a tutorial/guide that explains more about it:
http://projectsfromtech.blogspot.nl/201 … y8545.html
p.s. maybe there is a clever way to reduce your pin usage and free one up for another use.