If things get to tight I just put furnace on R/A stand and come in the bottom.(providing you have the height on plenium. You will get more free area space than going in the side. You can put filter between stand and furnace bottom.
I find you also can put radius boot with turning vane on bottom of R/A drop for laminare flow. Looks nice and if you ever have to service i is nice not having to be on knees.