The shop I worked at for Rider Truck was a shipping depot, we would get 5 or 6 366's a week and ship them out to other dealers. This was in the early 90's. GM tried to save money and removed the upper bushing in the distributor. The housings were wearing out causing a misfire on the back cylinders. Most of the cores coming through had holes blown through a piston or two.
You might check the play in the distributor shaft if you find nothing else wrong.
Joe
