[Straydog]

I f your valve guides are getting worn and old trick I used was to install Ford small block umbrella valve guide seals instead of the o rings that gm uses.You can do this with the head still installed,by using a fitting that screws into the spark plug hole like a compression guage line. hook your hose from your compressor up to it and pressurize the cylinder. this will hold the valves in place while you remove the valve springs and replace the seals.
ONE CYLINDER AT A TIME.

Plugged drain holes as Tim Cline said along with loose guides and or bad valve guide seals will cause intermittent smoking.

Oh! make sure both valves are up (closed) before you pressurize the cylinder...