I would highly suggest getting a OEM heater core. I bought a replacement from 50resto.com and compared to the ford part, it's not even close. I changed it out before I had a problem, and the OEM core looked near perfect. It is WAY better built, so much so, I considered putting the 10 year old OEM one in instead of the new replacement. I should have returned the replacement and got an OEM... don't want to do this job again soon...
FWIW - while the dash is out is a great time to install a clutch quadrant... super easy!
Rick