318 running rough...

For the past 2 years I've been restoring a '67 Barracuda. The guy I purchased it from claimed that it had a 273 that was changed out for a '68 318. How many miles he had no idea. I recently rebuilt the carb and installed an electronic ignition. Afterwards, it ran very rough. I tried to adjust the timing, but noticed the line on the HB was jumping around. Figuring it to be the timing chain, I replaced it. Gears looked fine, the chain was pretty loose (I did find out that the engine is a later year than '68, due to the gasket on the pan end).
Started it back up thinking I'd find an improvement, but the timing is still jumping. Any thoughts?