I have a set of both 318 and 360 stock manifolds. The port size of the 318 are vastly smaller. If you're dead set on running stock manifolds, find a 360 or 340 set. Can get those cheap, usually. Other wise go headers.
The sheer small size is gonna limit your power greatly, especially if you're doing any type of compression bump. My guess is you'd be lucky to break 300 on stock manifolds