Running rough for no reason
 

I have a 1966 ford ranchero with a 302 and an hei distributor. Its running rough everywhere but idle and it feels like I've tried everything to fix it. Have checked timing, changed coil pack on top of the distributor, ran a compression test on all 8, changed all 8 plugs, checked valve lash, restock cash isn't clogged or flooding. There is one problem though, is overcharging to the tune of 18 volts. It has an internally regulated alternator off of a Chevy which test just fine at an auto parts store.