Good question.
I´ve been searching for my 75 Ramcharger too, and wasn´t able to locate any rubber sets so far. Only the Polyurethan stuff is available from Energy Suspension and one more supplier....
I bought the set from Energy Suspension but haven´t mounted it yet.

I came across two websites that offer rubber stuff, but not specifically for trucks.
At least they have listed the dimensions in their descriptions, so it would be a challenge to get the needed stuff together....

Have a look:

http://stores.restorationspecialties.com/body-mounts-solid-rubber/

http://www.steelerubber.com/body-mount-pad-fittings