It would be a good idea to start with a compression and/or a cylinder leakage test to see what is going on inside.
Then: don't overlook the obvious. If you are chasing a bad running engine, make sure the tune up is good first. You could throw a $1000 worth of parts at it and still have a misfire because of a bad cracked plug or a carbon tracked cap. What ignition system do you have? Contact
@halifaxhops for some quality parts if you are running a stock system. Follow up with the correct heat range of plugs and a good set of wires. No high dollar high performance ones needed, just a quality set. Then you can start chasing ghosts.
Oh and don't forget a proper valve adjustment. 273's came with solid lifter cams and need the valves adjusted from time to time.
