Is the basement floor carpeted and padded?? or bare concrete?? If carpeted, it will provide some insulation from the cold concrete, but a duct right up next to the fan inlet is by far the best.. You can use two 4" dryer vent ducts one from each end of your upstairs,, keep the 4" above the suspended ceiling.. until behind the new Mag stoker.. then 'Tee' them together and drop the single duct to just in front of the distribution fan's inlet.. it will help a lot to pull some air from the extreem ends of that long ranch house.. otherwise I suspect you will have cool to cold rooms at the ends of the house.. It really doesn't take a lot of circulation when it is 24/7. The bigger circulation fans on Forced Air systems are meant to run only when heat is called for, and to quickly bring the temp up to target,, if a little air is moving 100% of the time, it is more comfortable, because of no drafts... Only when the outside temps drop way down will you maybe need more circulation..
As for dumping the hot air above the suspended ceiling, using it as a large plenum.. are the ends of the joists insulated? If not you will be wasting a lot of heat at the joist ends and perimeter of the deck.. I think you will be better off with ducts..if you can make them work.