They didn't spend crap on developing the 3.9, go read the story at Allpar, it will answer your questions.
Yeah I'm sure casting a new block, crank, heads, cam and associated R&D (even if robbing as much 318 stuff as possible) and topping it off with a carb that was obsolete before it was ever introduced on the 3.9 was cheaper than installing one of the other off-the-shelf EFI engines they were already producing....
Chrysler pulled the plug on the Slant because they simply couldn't make it run clean enough for the tightening emissions standards.
Surely if they made a new head for the slant and upgraded the induction system it wouldn't have been any different from any other engine? AMC seemed to get by with their carbed straight 258ci I-6. Jeep wranglers still came with the carbed straight 6 until 1991 and that engine dates back to as old as the slant.
