New heater core problem..

[HIGHWAY BY THE SEA]

I have been fixing my heater system in the 67 GMC (yeah I know it is summer) and have hit a snag. I went to the local parts store that I always use, and found that the heater core they sold me (aluminum) will NOT fit with the two mounting brackets.
The sides of the core that the brackets hold down are too fat to fit. Has anyone lese noticed this issue with new heater cores? The parts store was NOT able to get any other type either.
Is there some new type of fatter mounting brackets that hold the heater cores in place?