I'm guestimating 15's-16's. But that's because I haven't yet swapped my rear out. I have a 8 3/4 with 355's that I need to take the time to install in the next few weeks.
Start watching this member here and follow along to see how he tunes his 318 to run very well. He is currently starting on a 5.9. He makes the most and best of his engines to go fast cheaply. This should be a good start for you.