Clean the bowl area up, reduce and narrow the guide boss, radius and smooth the short turn radius. You can use the round port header gasket for a template to make a quasi- D port exhaust. I would just use the smaller round port header gasket, scribe it and just grind the roof area larger, and concentrate on the port roof and sides, but staying away from the exhaust port floor. If you want, you can make the roof area a little larger than the stock round port header gasket, but I wouldn't go as large as the bigger round port gasket.
Be careful, Vortec castings are thin, and hogging them out leaves them very fragile, and you risk grinding through the port.
You can pickup an easy 20 CFM if you are careful, and you won't have to remove alot of material.