Put a little bigger cam in it and bleed off some cylinder pressure. That should get it to run reasonably well on pump gas and still have enough spark advance. That's assuming the rest of the engine is in good shape...
Mine ran like crap on 93 octane pump gas with all stock innards. Retarding the timing helped somewhat, but it was never right.
If you bail on the 340, I'm sure you'll have no trouble selling it.