You do not have to remove the heads but you will need to pump air into the cylinder to keep valve from falling into cylinder when you remove the spring keeper.
This is doable but there are issues.
1. You have to hold that piston at TDC so that the cam is not holding a valve open.
2. You use any sort of fitting (a compression tester will do nicely as it will probably mate right up to a quick-disconnect on your air compressor hose) to fit the spark plug hole. You need to turn on the air and keep it on, but not 150psi. ideally 30psi or so would be nice. Too much and the motor can turn over if you are not careful, which will let a valve open, lose the air pressure, and plonk, down the valve goes into the cylinder.
3. It is also pretty inconvenient to work on the heads while on the car, you have to have a special valve-spring compressor that uses the cam as a fulcrum, and fiddling with those little keepers while laying over the fender and working on the back valves is a chore.
If you take your time, and make sure you cover the area you are working around so you don't end up with a dozen keepers in the oil pan, then this is easy. Once you remove the keepers with a magnet, the retainer, spring and seal can be removed by finger pressure alone. The interesting part is putting it back together. The seal is easy, it just sits in there, but getting the valve spring and keeper back on, then compressing it and fiddling with the keepers (keep some grease handy so they will stick to the valve stem once in the right position) can be a real pain in the a**.
