did this id/od changeout with two others at work. The old unit was behind the beam toward the front of house, away from access door. Dug out so new plenum woud fit under beam and offset duct to tap into plenum. Much easier access now, just step in and you're there.
Old supply plenum butted to bottom of trunk line before. Did plenty of sealing in the return head too, (exposed wall cavity, restricted hole through floor, crappy connection to duct).