Reply to "spark timing"

I don't think the misfires are distributor related now.
You could look at all of the plugs and see what they look like.
You could be blowing carbon off of them that has built up on them and the valves.
You need to drive it but don't just take it out and go 150 for a couple of miles.
If it is carbon then that will just melt it and it will appear as a glaze over everything.

You now need to examine all of the vacuum plumbing and look for leaks.

Look at the brake booster too. You need to test it to see if it holds pressure. It could have a small leak in the diaphram and the misfires could be coming from a vacuum leak somewhere in the system.

If all of that checks out then it may be time to pull off the heads and reseat the valves.

These valves are not like the current 4 valve per cylinder overhead cam engines.

Those engines will go several hundred thousand miles without any need to reseat the valves.

The Cleveland really needs to be looked at for that around 70 to 80 thousand miles.