spectra premium heater core issue ? / anyone recommend a good heater core ?

ok , here is what happened so far : last year heater core failed in my 1974 valiant 225 /6 . i picked up a used stock one at carlisle just to get by . which it did get me through the winter , and had hot mega-heat . the kind you got to turn down after awhile . this week the used one pee'd on my friends leg on a short trip to get lunch . i went to autozone and bought a new heater core ( spectra premium ) and installed . just barely got warm . plenty of coolant in radiator . both hoses to core got hot . upper rad hose hot . everything else i believe went back in as it came out , cables , wires etc . cables are working properly. blower motor/fan is blowing plenty of air .

next , with the fresh air door open on passenger side and the heater control on low ( which opens the door so you can get your hand in and touch the heater core . blower motor/fan switched off , of course . don't put your hand in there with moving parts ) so , i touched that part of the heater core and it was cold . this was after driving around for 10 minutes with heater on full blast .

next , i fired up the 72 dart 318 which was cold . and did the same thing . heater core very hot to the touch within a very few minutes just sitting there . and of course hot heat .

i then pulled out the whole heater box from the valiant , removed the spectra heater core from it . and blew some air from the compressor at 20 psi and got good flow of air coming out both tubes , both ways . then i connected it directly to the heater hoses . both inlet and outlet tubes got too hot to hold but center/ rows part was almost cool to the touch at first , then started to warm up after 5 minutes or so , but still never got too hot to touch or like you can hold your hand with force on it for more than several seconds . though it seemed like the back part of it was hotter than the front part which faces the blower motor/fan . by the way the spectra core appears to be all aluminum .

then i connected the old used one to the heater hoses ( which is brass and i am guessing copper or steel ? ) and the tubes were to hot to hold almost instantly , and the center / rows parts got too hot to touch within a minute or 3 .

did anyone else experience the same problem ? is this spectra thing just happen to be defected . or made crappy ? should i return this for another one ? or am i missing something here ?

anyone know where i can get a good and / or a brass type heater core ? or have any ideas on what is happening here ?

also , some more info , my temp gauge is not working , but the hoses got very hot .