You are going to have to remove the valve springs and the valve keepers to get to the valve stem to install the seals
That requires a valve spring compression tool that can only be used with the heads off —as far as I know
Some seals ( pc seals ) require that the heads be cut at a machine shop to fit them ( used them on my race engine with great results )
If the compression shows good results —-I would pull the heads and have a machine shop do what they do to bring them back to life and install new seals
You may want to upgrade your valve guides to the harder ones—- ( if not done ) to be used with the ethanol fuel most of us need to be using these days
New valve guides will help by preventing oil that is getting into the cylinders —-down worn valve guides
You can use compressed air thru the spark plug hole to hold the valves up so they do not fall into the cylinder but not sure how you will be able to remove the valve springs to get to the valve stems with the heads on the car
If you can have the heads done do it to building reliability and performance into your engine
Just my 2 cents — hope that helps some
They use replacement hardened valve seats in the heads as well for ethanol fuels
Fresh heads on an engine is a very nice thing to have— in your pride and joy
Get a spring compressor, a engine compression kit with the air fitting for the spark plug hole and a good solid compressor.
Use the air fitting in the spark plug hole to hook up an air hose. Only take one spark plug out at a time so the engine does not rotate. Have the piston at TDC also as a precaution. The air pressure keeps the valve closed while you do surgery.
Do you have a method to be removing the valve spring with the heads on ?
I have replaced broken valve springs with the air method but always had the heads off —to pull all the valve springs off an entire head
You can change the valve seals with a regular valve spring tool and filling the cylinder with compressed air to prevent the valve from falling in the cylinder. Don’t bother trying the flat bar method that they show in the manual, it is not easy. The valve seals in our cars are not a positive seal, they’re just a little o ring wrapped around the valve stem to prevent oil from dripping down.