I ran into the same problem with the vertical mounted radiator in my 74 Argosy. If your radiator is mounted vertically between the frame rails then as you've already found those pads are no longer available.
I ended up fabricating custom rubber pads
. If you look at post 92 in the link you'll see what I did to solve the problem. They worked out very well and the radiator is sitting at the correct height although I think I did add a couple of washers to shim it just a little higher.
Post 835 at this link
shows the radiator installed.
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)