what size tires on my 64?

I'm going to get new ones this week but cant decide what size, its a coupe with newer 14 in styled steel wheels. I put the one inch lowering springs a long time ago so it sits level, I also put 2:80 gears in it but I want a little taller tire cause it still rpms a little too much for me on the freeway. right now it has 205 60 14's, should I just raise the series to a 70? would that be enough?
[addsig]