Aftermarket parts store stuff can be made to work, but it doesn't fit like original.
On my E body AC heater cores, the flat plate the mounts the Heater core to the box wasn't shaped the same or soldered on in the correct spot.
Took a little work to get it to fit.
Not much work, but it didn't just fall into place like it should have.
On my 70 Roadrunner, I ended up taking the aftermarket heater core to a local radiator shop to have them adjust the location of one of the tubes and resolder it.
Now it works, but if you can save yourself the headache of hassling with it, do it.
