I apologize - after rereading my post I realized it sounded very critical and unsupportive, not at all what I was trying to convey. If you can find something that fits and works well in the factory location and is cheaper and more commonly available than the original part, that's a benefit to the whole community, and I applaud you.

I've replaced numerous heater cores over the years and they have all been fairly specifically engineered for their application due to packaging requirements under the dash. Most of them have been tricky enough to replace with one designed for the vehicle, much less one that isn't even supposed to fit in that space.