I am looking for a manual or explanation on how to rebuild the non A/C heater box.
I have all the pieces but I don't know how to put them toghether... 
Can somebody post pictures or diagrams on how to build it? I am pretty shore that somebody should have documented his restoration...
I don't know if there have been any threads that addressed this topic or not. I have restored a number of these. If there are no threads available, the best way I could say it is to put it back together like it came apart. I am not trying to be funny. Seriously, take lots of pictures prior to disassembling and during the process of dismantling.
Just some notes from what you have written so far that you will need. High temperature spray adhesive for the inside foam gaskets (I use 3M 90 spray adhesive) and rope gasket material for both sides of the new heater core. Also, some of the new heater cores need the metal mounting plate cut a bit to get it to fit into the box.
The two biggest pieces of advice is to take some of the spring out of the large metal clips. The new ones are so tight that they can break off the mounting tab. Stick the small rounded end in a vise and push the other side of the clip backwards. You want just enough to close the two halves together and keep them tight. Put the small circled end of the large clip in place first and use a slotted screw driver in the other side to help set it in place.
Second is when re-installing, keep that intake hook very loose. Once in place and bolted from the firewall side, then tighten the hook.
I have some restored and disassembled if you need some help and pictures along the way.
Slotts
