Ernie, that looks like a "pickle fork" we used to use on tie-rods and ball joints. Good thing you found a second use for it.
Mike, as you are doing this, you might want to rotate the engine for each chamber so it is closed. That way the air tool will hold the valve closed. Without it, the valve will drop into the chamber, on top of the piston.
This means you should do all of the things you want to do to the spring at one time.
put compressed air in chamber
Compress spring.
Pull locks
Do whatever.
Put locks back on.
remove comressed air.
next chamber.
Rick's idea to make the car safe for work, will need to be modified. You might want chok the tires so you can turn the engine over with a breaker bar ( at the crank ). You can also use the breaker bar to keep the piston from moving to the bottom of it's stroke.
Hope this helps.