Unfortunately the 74 Argosy is totally different than the 345. I really wish they were the same!
As Winestream said I'm in the middle of tearing most of that out of my 74 Argosy. With advice from John (Shepherd57) I've managed to get the dash top off, and I've just about got the top aluminum plate off that covers the air plenum. If you check out the link that Winestream posted you'll see where I'm at.
What I don't know as yet is whether all of the above has to be done to get to the heater core. John might be able to answer that question. I'm removing all of the system because the vacuum actuators that control the dampers on the plenum are frozen and there are vacuum actuators located inside that need to be replaced.
Have you looked on the engine side of the heater box? I recall seeing an access panel that should be accessible from the engine compartment.
Worst case, if you can hang on for about 3 weeks I will be back from a trip and will be pulling the aluminum panel of the top of the dash and will continue with tearing into the heater system. I'll be posting pictures at that time.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)