Scott,
Spanky would most likely know the answer to your question.
As you probably already know
and you can see from these frame diagrams, the COE cab sits up on a "platform-frame" above the lower frame rails.
If you do not have a 54/55 Truck Data Book, I could post a few more drawings but I do not know how much they would help to answer your question. Just ask, if you want them posted.
From the diagrams at the link above, you can see that the COE cab would have to attach well above the 6500 frame and the COE inner fenders (attached to the cab) would also then have to attach-to (line-up properly with) the radiator housing, which would be attached to the lower frame-chassis.
Tim
