Put each lifter in a cup of oil (submerged) and push down on the plunger (use a push rod) with the lifter under the oil. That is really all that is required. The lifters will self-prime anyway so you really do not have to do anything. If you cannot push the plunger down that is good. Do not depress the plunger unless the lifter is in oil as that would force air into it.
The valve stem seals are likely fine. It would be OK to leave the valve springs alone.
The pushrods are fine. Reuse them.
It sounds like the rocker arms on the cylinder with the failed lifters may have come loose causing the failure. Did you unbolt the rocker arms from the support bar? If so, put the rocker arms back on the bar and install the entire assembly as one piece instead of bolting on the rocker arms after the bar is in place. The unit is designed to go on as an assembly. Individually assemblying the rocker arms after the bar is in place will likely strip the rocker arm bolt holes and/or break the rocker arm pivots.
Go back to the GM dealer and buy a new distributor gear for the distributor. That mates to the cam gear and you do not want to put the old used gear on a new cam. Install the new gear with a moly paste for breakin protection.
While you are at the dealer, buy a quart of GM EOS (engine oil supplement) and pour it into the crankcase in place of one quart of oil when you refill it. The EOS is heavily fortified for antiwear protection and will help the new cam break in correctly with no wear. This is important.
Use the heavy duty Delvac, Delo or Rotella oil in the crankcase. Cheap insurance against wear and the best oil for that engine.
Use the GM coolant supplement (sealer) in the coolant to prevent any coolant intrusion into the crankcase in the future.
If you are not putting a new timing chain and gear in....stop, go get one and replace the chain and gear when you reassemble. The cam gear in that engine had plastic teeth and the teeth will fatigue and fail. Even if it is fine now it will fail in the future so replace it.
The valve stem seals are likely fine. It would be OK to leave the valve springs alone.
The pushrods are fine. Reuse them.
It sounds like the rocker arms on the cylinder with the failed lifters may have come loose causing the failure. Did you unbolt the rocker arms from the support bar? If so, put the rocker arms back on the bar and install the entire assembly as one piece instead of bolting on the rocker arms after the bar is in place. The unit is designed to go on as an assembly. Individually assemblying the rocker arms after the bar is in place will likely strip the rocker arm bolt holes and/or break the rocker arm pivots.
Go back to the GM dealer and buy a new distributor gear for the distributor. That mates to the cam gear and you do not want to put the old used gear on a new cam. Install the new gear with a moly paste for breakin protection.
While you are at the dealer, buy a quart of GM EOS (engine oil supplement) and pour it into the crankcase in place of one quart of oil when you refill it. The EOS is heavily fortified for antiwear protection and will help the new cam break in correctly with no wear. This is important.
Use the heavy duty Delvac, Delo or Rotella oil in the crankcase. Cheap insurance against wear and the best oil for that engine.
Use the GM coolant supplement (sealer) in the coolant to prevent any coolant intrusion into the crankcase in the future.
If you are not putting a new timing chain and gear in....stop, go get one and replace the chain and gear when you reassemble. The cam gear in that engine had plastic teeth and the teeth will fatigue and fail. Even if it is fine now it will fail in the future so replace it.