On mine, I pulled the entire unit out - including the sheetmetal enclosure.  I think if I had tried to just pull the furnace out and leave the enclosure, it would have hung up on all manner of things.  Once the whole unit is out, you can readily overcome these and separate the furnace out of the sheetmetal enclosure.  While it's out, look carefully at the tubes for any cracks.

Thanks for the tips. I did find a video: https://youtube.com/playlist?list=PL66r7Hir5gawRxBChC8lwUyVi6_YRDFJO&si=-y6Ru0fBNi6y6pqX

Unscrew the unit’s exterior trim panel and you will see the furnace itself; there will be mounting screws that secure it to the surrounding framing or carpentry. There should also  be some visible 12V wires to disconnect and the propane line as well. Once all that is free, you yank away at the whole chassis and hope the exhaust pipe isn’t frozen to the ancient, rusty external vent!

That’s how I remember it anyway.