trying to find a replacement heater core that FITS!

Trying to replace the heater core in my 67 Dart (no ac). Tried a readi air and one tube hit the top of the slot through the heater box, tried tweeking the tubes slightly and broke the solder joints at the heater core. Ordered a Spectrum Premium from Rock Auto and am having the same problem. The readi air looked like it had crappy solder joints to begin with, the spectrum is aluminum but looks like it has better solder joints. Any tips or comments?
Thanks
Aaron
The heater core had a small leak. The stock heater core that I am replacing drops in and sits perfect