[LS short box]

Ok one more thing. This a bit of a "Bubba" fix thing but I use it all the time when I'm changing valve springs on a LS build. In lieu of using compressed air to hold the valve up while I replace the valve spring I thread in thru the spark plug hole small length of cloth line rope and the bring the piston up against the rope to hold the valve in place while I replace the valve spring. Works great.
So in lieu of a proper piston stop the the cloth line rope could be a cheap piston stop. Not quite as accurate as real hard stop piston stop but it will get you close.