On Piston to head clearance about .040 w/steel rods.
Do not want big lift, must flow over valves.
Mill head only to make flat.
Valve to chamber top as large as possible. But not with thick gasket.
DO NOT relieve block, hopefully valves are even or lower than block surface, put a small radius on the cylinder edge in the combustion chamber area only, about 1/8 to 3/16. helps to turn air fuel into cylinder, not sharp edge.
Do you know cam lift and duration, if not check it out, and the separation angle between intake and exhaust lobe on the same cylinder [ this is very important on a turbo engine].
Important note: with boost need more spring pressure because of boot pressure on back side of intake valve.
Too bad on only 4 intake ports, depends on port layout and valve timing events.
Check cc of large chamber and small chamber want large transfer area but try to keep compression at 8-1, but transfer area is most important.
Built two turbo Flatheads 1 race 514 HP 640 FT LBS *
1 street 350+ HP
* Race turbo flow maxed out would have made more with larger turbo, Still set record at Bonneville at 212 MPH
