I have a spreadsheet that tells me the appropriate minimum cross section required. Pipe Max is also very helpful in this regard.

Some engine builder or cylinder head guys will just do the math.

That is just going to be the starting point though. Darin Morgan described himself as a velocity manager, rather than a cylinder head designer. I thought that was very apt.

As we are talking hyperthetically here, if you were wanting to use a BBM style head casting as an example, you would be trying to get a pro port style casting (very small ports, no guide work or chambers), or doing a lot of welding.