Resin solder is one of the better materials to use, but many have had success with some modeling clay (if you are careful). I had no issues. Just stick a 1/4" thick pad of the material you choose around the valve reliefs.
Typical suggestions are .080" on the intake and .100" on the exhaust. Some other valvetrain companies suggest to be even safer, at + .020-.025" to the previous suggestion.
Remember, peak lift has nothing to do with piston to valve clearance.