I just went through this a week back with the AMC V8 in my Jeep.

You can "make" an Air-holding tool/fitting, by using a Spark-plug extender (available at autoparts stores) and 1/4-inch air-tool quick-disconnect fitting, plus some teflon-tape to seal.



You screw this fitting into the sparkplug hole for the cylinder you are working on and use compressed-air to hold/keep the valves from dropping down into the cylinders while you compress the valve-spring and remove the valve-keepers.

An older, alternative method, involves feeding small-diameter Rope, through the spark-plug hole, filling-up the cylinder. When the valve drops inward, the rope keeps it from falling all the way into the cylinder where it's un-retrievable (else you'd need to remove the cylinder head to get it out.)

You will also need an "On-Engine Valve Spring Compressor," to remove each spring. I bought a "Good" one for $40 at NAPA. I (also) bought a crummier one for $20 at Pep-Boys, though with weaker valve-springs, the cheaper one would likely work fine.



TIP: You should "hit/tap/smack" the valve-retainers (I used a wide, deep socket and rubber mallet,) to free-up the Keepers, otherwise they (sometimes) stick/bind in the retainer opening.