Odd Ductwork Design

[Porcupinepuffer]

We're kind of scratching our head on this one. We have this oil furnace we need to yank out and replace with a gas furnace. Issue is how the ducting is designed here. First thing that catches our eye is all the finished drywall right up against and sealed around the supply/return plenums. Entire rooms ceiling and basement is all finished drywall ceiling. We managed to remove an opening and discover that the supply plenum is about half complete and has a main branch and several small take offs that just blow all the heat into this finished ceiling area... The joists are a semi-modern steel/wood type joists that basically allow the entire basement ceiling area to act as a giant supply since no joists are sealed off. They just cut out and put grills in the rooms and the pressure buildup in the entire floorspace allows the heat to flow out the grills to the rooms. The return is supposedly ducted in proper areas. The customer says the existing oil furnace heats the place fine.  We know we need to remove at least an inch of the drywall around the plenum to meet the required minimum clearances that the furnace asks for, but if we do this we now have a big opening where the heat will be blown back down from by the furnace since we're quite certain it will take the path of least resistance and not fill the ceiling area to give good flow to the vents for the rooms.  It's like a combination duct/poor mans main floor radiant heating system.
As far as I can see, there isn't any codes that seem to be a problem. I'm just wondering how someone else would go about this.