When I needed a heater core for my '54 I could get a pepop from Concours for around $110.00 plus shipping or have the local shop recore it for $150.00 I decided to go with the recore. Try to keep the local guys in business. But I've always had good luck with Concours so I'm sure the one they have will be fine.
If you have a good radiator shop near you , take them your old one and have them check and see if they can put your old tanks on a new core. I did this for my '51 Merc. I think it cost me about a hundred bucks back about 15 years ago
I just had mine recored as it had a hole in it. They just kept the sides and soldered the valve on in the open position. This taking out a potential leak point. I have had other people tell me an early mustang core will work.