I figured when you started asking about welding rod the choice to buy heads was already made
![Smile :)](./images/smilies/icon_e_smile.gif)
I use a simple formula for seats given to me by another buddy...seat ID=0.86 x valve OD then try to make the seat the choke point in the port and the port tapers out at about 2 degrees included angle. Do that and it will always work well form what I can tell.
There are other ways to do it that work. The US car guy talk about bowl volume and such and I can't argue that it seems to work....but I'm pretty sure putting peak velocity as close to the cylinder as possible works better.
You need about 3cfm @10" or 5cfm@28" per hp you want to make. that gives you a flow target.
Then a good port will do right about 190ft/s velocity at 10" (328@28) really good ports hit 200, DO NOT accept a number below 180. This gives you the seat ID you need, from that you get the valve.
That's how I do it. You already kind of know what valves you are going to use, so go the other way around in the math and out will pop all the dimensions and the hp.
Edit: again, this is nothing I developed, a buddy who does engines told me it worked and damn if it doesn't.