Welcome to the Team John!
If they say it isn't the same it is probably different in some way. It may be that you could weld the two together to get one good 1964 Heater Core Box, Interior; but I don't know what the difference is (could be a missing bolt hole an extra bolt hole or a moved heater hose inlet location). No idea; but it was modified by Chevy to make it cheaper, or to solve a warranty issue (once again about money) that customers had filed complaints against the car. Those are the two motivators for change in the company.
I might add that with the steel tariffs and tariffs on anything coming from China (where 98% of all restoration car parts originate) a lot of parts and restoration warehouses are going to disappear; just like a lot of speed shops went belly up in the great recession. This hobby depends upon discretionary funds. When it gets too expensive people stop buying. People stop buying and the companies that sold them parts go out of business. Takes a generation to restart the cycle. When there are no bolt in parts you have to fabricate your own from flat sheet metal with a Planish Hammer, English Wheel, brakes and shears plus the good old TIG/MIG welder.
As a Post Script I might add that the Assembly manual will have an illustration of the part (you would need a 1963 Assembly manual to compare the picture to to see what is different).
To do any restoration you need a full set of the published manuals from Helm that GM gave to all of their techs every year. You will need an Assembly manual: shows a drawing of the part, list the then valid Chevrolet part number (they constantly change with every modification of the part), any gaskets or sealers used to install it, or the bolt torque, bolt size (length, thread pitch, and grade), and quantity of bolts used (if any), and the order of installation.
You will also need a Service manual that tells you how to diagnose problems with the power train and how to R&R every part.
Further the Chassis Service manual details frame and suspension parts and includes the specifications of all bolt torques, fluids, capacities, and service parts to maintain your car.
The Fisher Body Service manual was written for the collision tech to repair by removing and replacing every body panel, window glass and door or trunk panel, and exterior trim piece on your car (great to find out where to drill holes to mount all that missing SS trim).
Last edited by Big Dave; 07-15-2018 at 03:32 PM.