340 valve springs

Take a spark plug to the hardware store with you.

Find an air fitting or adapter with the same thread.

Clamp a hose on it with the fitting to hook up to your air compressor on the other end.

Do one cylinder at a time, making sure to turn the engine until the exhaust just closes (you are then on the base circle for that cylinder) and then pressurize the cylinder.

Use a typical inexpensive spring compressor (screw and hooks type, should be available at local auto parts store).

Keep a few magnets around the valve area and put clean rags in the cylinder head openings to avoid any keeprs or retainers getting dropped in.