78 country squire engine misfire

In cleaning it up, you might be better served to pull it and take it over to a machine shop to get it tanked in order to remove all the carbon and any corrosion. Did any of the studs get damaged? They are supposed to stay in the manifold. If any are, measure one and let me know; I may have spares. Also, either I didn't ask or I forgot to--is your carb 2 or 4-barrel? I have a few suggestions, depending on what you have. Tell me in a conversation, so that we don't further hijack this thread....

 

On mine,I reconditioned the EGR spacer and blocked it off with a '72 gasket. The actual cause of all these blues was a defective distributor with a floppy plate. Remanufactured,ugh. I got a new one,everything works fine.

 

if you have one of those temperature reading guns point it at the exhaust for the individual cylinders coolest temp is probably where the misfire is . not sure if it will help but just a friendly tip someone passed along to me

 

He's got a good tip there. You don't look for specific temps, but you do look for the one that's lower than the others, similar to doing a compression test.

 

Sure seems like ruptured diaphragms on all three valves. This will cause a vacuum leak and could be your miss(sounds like a lean miss), have to tried simply blocking off the EGR and plugging the vacuum hose? An EGR hanging open from carbon build up can also cause a lean miss.
I think you are on the right track with the EGR system.