cleaning up the slant six head

There should be some boundary layer in the intake ports, it shouldn't be entirely laminar flow. The reason is to provide enough turbulence to pick up any liquid fuel and re-introduce it to the main flow so that you're not dropping liquid fuel into the combustion chamber. The way to create that boundary layer is to have some surface roughness on the port walls. The rougher the finish presumably the bigger the boundary layer, but that is not something easily controlled or predicted. So just go with rough, not polished and call it good enough.

With dry flow, like in an EFI intake above the injectors or in exhaust, you don't want any boundary layer because it serves no purpose and reduces flow. So those areas can be polished smooth. Extrude-Hone type processes do a marvelous job of this. It's basically abrasive Play-doh (from what I can tell) pumped thru whatever under some pretty good pressure. It won't really change the shape of the port, but it will keep taking off the high points until the port walls are fairly smooth and the successive cross-sections are very uniform and consistent.