I used a spring compressor loaned me by autozone. It worked well except the narrow bottom spacing of the variable rate springs caused a bit or a problem. This compressor pressed down on the valve retainer and loosened the keepers. I happened to have a small rod mounted magnet made to fish dropped nuts out of inaccessible places. It worked wonderfully for fishing the keepers out of the retainer. Also worked well for dropping them back in place after installing the umbrell seals.
The compressor I used would not allow me to do the back (exhaust ) valve which is somewhat recessed under the firewall. There is an argument made for not even installing umbrella seals on the exhaust valves.
I'm told that the original factory valve seals (the o rings) only serve to prevent drain down of oil collected in the retainer cup when the engine is stopped. That collected oil would otherwise trickle down through the keepers and down the stems into the combustion area to produce smoke for a minute upon startup.
meansam

1960 Chevy Stepside