1968 Airbox rebuild question

The new Heater core is a Spectra. Their website, and Classic Industries lists this as the proper one. I compared it to the Replacement copper one I took out and discovered this: The heater cores look identical but, the right tube, facing from inside the box, drops into place. The same tube for the spectra sits too high. The only way for that tube to clear the hole is to twist the metal plate, up at an angle, which it did, tearing out the center plenum. So even though Spectra lists this as the correct replacement, it does not fit. Can anyone suggest another good source for heater cores. Year One has them but we're talking maybe $100. Or do I spend the money with them? Thanks