Those 40+yr old ducts are dry & brittle. Don't take much to render them useless. I replaced mine when I replaced/restored my heat/ac box w/ new 2.5in ducts from www.nostalgicairparts.com.
yeah mine are like dry rotting or something a little, but luckly i was gentle enough with the metal tabs, and got it in right
I'm needing to change the heater core in my 73 mav. It came with factory air but not under the hood anymore. I'm dreading it. Looks like a lot of work.