Unless your panning to add power soon I would leave it stock. I have a plus 4, running big 3 with duals and a bored throttle body. Seems perfect. Unless you can't keep the front end down leave it stock.
There is a lot of factors that go into that including the type of paddles your running. My build with a p&p head w/+1 valves, webb 4, 14:1, and barkers I ran a +6 arm. I feel like a +4 would have been better. I could sit on the back of the seat and it would keep the front planted running 8 paddle haulers. On the stock arm I could not keep the front end down at all.