The original genset would likely have been a 4K unit of either Onan or Generac manufacture.
The odds are the genset is installed from below which is how my 20' front bath model is set up. Even the 345 was installed from below. However our 310 was set up to come out from the side or drop down from below. Personally I like the ability to remove from the side which means it can be put on drawer slides for easy access and maintenance. That's what I did on our 310 and will be doing on the Argosy.
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)