Forgive me if this has been mentioned before, but have you checked your vacuum advance??? If they are not working properly, engines run hotter, lose some performance, and get even worse mileage!

One reason I like GM luxury cars of that era, like a 1971 Caddy I had for a long time...Idiot lights!
Hot light, then if you really push it, there was the stop engine light!
Had no idea how hot it was running, Hot light only came on once or twice on 98 degree days after stopping for gas.
After having a 1973 New Yorker, it was nice to not forever watch and worry over the Temp gauge, with the A/C on! Something to be said for being an idiot.