You can smooth the valve stem support in the exhaust port to streamline it but I would not remove it, this supports your valve and also gives more cooling area for the valve stem. Match the gaskets and open up the area adjacent to the valves in the combustion chamber to bore size (use your head gaskets and scribe), this allows more flow around the periphery of the valve head and reduces the restriction. Don't square up the wall just lay it back a little up to bore size.
Blend the area inside the port near the valve and remove any sharp casting ridges. If the valves seats need to be done then back cut the valve heads using a lathe or in a pinch chuck the valve in a drill press and back cut using a die grinder. This is best done before the seats are cut or lapped because you'll "kiss" the seat faces for sure. Use aluminum tape to protect the seats during your grinding, use at least three layers.
The short side radius should be where you spend most of your time and make sure you don't get too crazy on the first cylinder, you will have 7 more to do. Wear safety goggles under your full size face shield and use a vacuum to pick up shavings, not compressed air. Tape your sleeves and neck to keep them out of your clothes.
One last thing, forget polishing anything. Rough surfaces flow more air-that's a fact. A 60 grit cartridge roll should be the finest abrasive you use.
She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.