Anyone know of a diagram or photos of how the heating tubes run from the fan mounted on the firewall to the heater core under the driver's seat? I can't remember how they went from when I removed them. Thanks for the help.

Here is what I found in the 1955 Pontiac Shop Manual 

You are in luck that this 1955 shop manual is available online....you might want to bookmark this link Mike.

http://pontiac.oldcarmanualproject.com/

http://pontiac.oldcarmanualproject.com/manuals/1955/1955 Pontiac Shop Manual/14-Accessories/image6.html

Thanks but what I'm after is do the tubes go through the holes in the frame, go under the frame, inside or outside?  Should have taken photos.

I'm not sure if this is any better for you. I found this illustration in the 1949-1954 Pontiac Shop Manual. There are no other illustrations either in the '56 and '57 manuals.

I'm making a trip to see another 55 and find out the routing of the tubes, snag a few small parts and answer a few other questions about how things go together that I have on Monday.

I'll be curious to know the answer myself. Take lots of photos or video.