checking valve to piston clearance with hyd lifters?

I need to check my valve to piston clearance in the 360 Mag I am building right now. I was going to use the clay method. If I am doing this with hydraulic lifters, how can I ensure an accurate measurement? Is there a way to ensure that the lifters are fully pumped up before proceeding?

Thanks,

CE

Oh, and anyone know what the min clearances are for int and exh on a magnum engine?