Determining the maximum valve lift can be a little tricky. I usually do a trial assembly on any engine where valve/piston contact is a concern, with the lifters, pushrods, valves, and piston installed on one cylinder. The critical point is on the "overlap" position between the exhaust and intake strokes, when the exhaust is closing and the intake is opening. Stovebolt exhaust valves cannot get anywhere close to the pistons due to the combustion chamber design, so the intake is the only one to be concerned with. I use a very weak spring on the valve, one that can be pushed down by hand, and find top dead center on the exhaust stroke. Then I use a dial indicator to monitor valve movement as I push the valve down to contact the piston. It's a good idea to maintain a minimum of .060" clearance at the closest piston/valve relationship. It's possible for a good machinist to recess the valves deeper into the head to increase the safety margin, but there's a possibility of restricting gas flow by shrouding the valve if it's recessed too far. Finding a really good machinist with some racing head prep experience is definitely a good idea!
Stronger valve springs are only needed if you plan to run very high RPM, and/or run a cam with a very radical rate of lift.
Jerry
