Howdy.
For a good source of parts try LMC truck catalog CBE.33 for 1960 thru 1966 Chevy trucks, page 35. You can find the core and all of the other components. You can see the catalog on line at
www.LMCtruck.com and order a free catalog from there also. I have rebuilt two heater systems using LMC Truck catalog without any problems.
For a drawing get yourself the 1960 Chevrolet truck shop manual. I found mine at a swap meet. The heater section starts at page 15-37 in the manual.
Hope this helps.
EDIT: Is the heater motor working? I would unhook the wire going to the heater motor and check it with a meter to see if any juice is going to the motor. Make sure the heater switch is working. Check the defrost hoses for splits and tears. Check to make sure the cable assemblies are working properly, etc. Better to order and fix everything all at once!
Iron Clad