1998 GMC C3500 leaking heater core

Last october one of our work trucks blew a heater hose and the guy driving the truck drove it untill it ran so hot it would not run anymore. When we got the truck back it would start but it ran bad like it needed a tune up. We pulled the heads off and the #1 clyinder exhaust valve was bent we sent the heads out had them magnufluxed resurfaced and the valve repaired we put them back on and the truck ran perfect. Then about 4 months later the heater core started to leak we had it replaced about a week later it started leaking again he put a new one in then about two weeks later that one started leaking, we bypassed the heater core and drove the truck all summer it ran perfect no coolant loss no overheating. We just had the heater core replaced and right away it started leaking. The guy that changed the heater core says that the heads or the head gasket are bad causing to much pressure blowing the heater cores out. Does this sound right would it blow the heater core before it blew the 16lbs radiator cap or the aluminum / plastic 100000 mile radiator, or did the mechanic just get a bad group of heater core or just mess up installing them