I received this tip on replacing valve stem seals from an aircraft mechanic. To replace the seals with the head on, remove the rocker arms and rotate the crankshaft to place the piston near top dead center and stuff the cylinder with 1/4" clothesline through the spark plug hole. I used a thin screwdriver to do this. Then, rotate the crankshaft to bring the piston towards TDC to compress the clothesline up against the valves. You can then use your valve spring compressor to compress the spring and remove it without fear of the valve dropping into the cylinder.

I replaced the intake valve seals on my '56 235 yesterday and this technique worked great.

Finally '54