What’s the go-to heater core?

Hey guys,
I just put a heater core in my 68 barracuda (a nice copper one) I don’t remember where I got it I ordered it in 2022…

Anyways I replaced it and rebuilt the box, vacuum filled the cooling system-started the car and it’s already leaking after a reaching about 150 degrees and 2 2500-3000rpm revs.

So what has everyone here been using brand wise? Which one isn’t a giant pile?

Thanks!