[blykins]

You don't want to buy the next size up if they are 180cc.

A 180cc port is plenty big enough for a 289. My 347's make almost 550 hp with 185cc heads.

The goal in porting is to increase the velocity and flow, not make things bigger per se.

We can essentially make a port "huge" and it may not do anything but make the engine sluggish.

With that Dart head, there's not any reason why you shouldn't be making up over 475 hp. Camshafts have a lot to do with that and most guys tend to pick stuff out of the catalog, which is a big no-no. You end up with a Chevy cam. If you mean that your cam has 256° duration at .050" lift, then your engine should be making a ton of horsepower. My 289's with ported factory heads (200-210 cfm) make 440-450 hp with about 10° less duration than you and only .500" net lift at the valve.