Can't find a heater core that fits - 66 Barracuda

Neither the O'Reilly nor the NAPA replacement heater cores fit the heater box in my 66 Barracuda. Both look very close to the original but the pipes won't clear the holes enough to seat the core's frame.

Anyone have any suggestions for an aftermarket core they've used successfully?

I don't think the original one can be fixed - it looks pretty bad. It's a non-AC car.