I think you can remove the furnace from the cabinet - the cabinet being the part to which the ducts attach. Step 3 below seems key. Suburban says:
SF-20/25/30/35/42, SFV-20/25/30/35/42, SH-35/42 and SHD-2542
1. Remove the vent cap assembly from the outside access door by removing applicable screws, also the four (4)
screws retaining the door to the frame. Remove the door to gain access to the furnace.
2. Turn off gas and power, then disconnect gas and power supply at the furnace. Label wires as necessary.
3. Remove the (1) one tie down screw securing the chamber to the cabinet.
4. Remove the furnace from the cabinet by pulling chamber outward completely to gain access to all controls and
applicable parts.
5. To re-install, reverse this procedure.
6. Rewire applicable wires and perform a leak test on all fittings. Perform a drop pressure test.