how many were built with a slant 6 and 3 on a tree?

Based on some quick web research, it looks like that's a rare car, as only 906 of the 90,000+ 1968 Chargers came from the factory with the Slant 6 engine: http://www.theautochannel.com/vehicles/muscle/chrysler/chry_68_chrg.frame. But you'll need an interested buyer to make a restoration worth the time, and you may have a hard time finding a collector interested in powering a Charger with a 6-cylinder engine. Hope that helps!

I have loved and owned several Mopars over the years and never knew that a slant 6 was built until watching Graveyard Cars. Very interested fact.