This may not be what your looking for but I was very happy with the Powerheads CNC heads for the 289. With their heads you go from 105.1 to 171.9 CFM flow. That a 63% increase in flow. I'm not sure what the other manufactures produce.
The initial castings had a protrusion in the exhaust port due to a core shift during casting. It was a 1/16 ridge that could be taken out with a Dremel. Don't know if the problem is fixed yet or not, but World Products was aware of it. Other than that, good bang for your buck if you are going to use iron heads. My second choice would be, as stated above, the Power Heads.