fly4,
Can you take and post a picture of your installed heater box and hose connection, as well as the inner fender area where "dimples" mentioned by member "Gates" are in the original post?
I don't have a GMC book/diagram, but do have a 1957 Chevrolet Accessories Installation Reference Manual with diagrams and descriptions of the hose install that might be of use to answer your question.
Need your pictures to try and make sure what I provide is relevant.
Dan
