I'm using a heater-core from a '87-'94 Toyota Camry. The core itself is manufactured by Nippon Denso (Toyota subsiduary). This is important, as there is a Natrad equivalent which is a lower performing core. The core itself has a fin area of 22cm x 14cm, is 32mm thick, but has a very high fin per inch density of 24 FPI, whereas the DTek cores are more like 12 FPI, so although they are thicker, there's actually far less fin-air surface area.
The DTek cores offer around ~1.1m^2 of fin-air surface area, while the Camry core I use offer ~1.85m^2 of fin-air surface area. A Black Ice Xtreme in comparison offers around 0.75m^2, and a BI Pro around 0.6m^2.
The final setup looks like this:
