Hey guys! I am replacing a 29 year old furnace, both the blower was going out and heat exchange was breaking down (carbon and metal bits etc) and smoking up the house. Acquired a great deal on a new furnace locally but here is the caveat, the new furnace is 3.5" wider so it is not going to be a plug and play situation like I was hoping for. Attached is a photo of the old system, everything is now disconnected, photos taken etc. New furnace is a Goodman 2-stage 120,000 btu model. My question, is the base return absolutely necessary with the electronic air cleaner there? Can I place the new furnace on the ground and cut the left panel, and then build out plenum from the new wider furnace to connect to the existing damper handle above? What do I need to watch out for and/or consider in this situation?

Thank you!