What should the plenum temperature be on a modern furnace
The proper delta T for a heat pump is 20-30*F.  For a forced air fossil fuel furnace I would expect to see 35-70*. If the return air was 50* you could easily have a plenum temp of 95* and if you had really good air flow across the exchanger, the return could be 60* and still have that discharge temp.  As to a remedy there are simply too many factors involved to address through correspondence.   In a nut shell the furnace has to put-out more Bu's than the floor, walls and ceiling loose for any given period.