What's up Brooklyn, I don't know of anything else behind the dash to address. As far as the brand of heater core, due to the work involved in swapping the heater core out, I would install the OEM or better. I have no idea what brand is better than the other so for me it would be the Motorcraft one.
Rockauto.com sells the Motorcraft heater core (F4ZZ18476A) for $75.00. The knock offs are less than $30.00. You get what you pay for I reckon.
Maybe adding sound deadner on the firewall is something to do with the dash out.
GL, Gilly