joeyd-
Here is the one I have been using for my first water cooling. It is big 9 x 6 1/8 core size actual height is 11 1/2".
This is a 1985 Ford Econoline
Heavy Duty heater core I picked up at Auto Zone. It was $27.99 plus tax.
I got this one over the Chevy Caprice 1975 since the outlets on the Ford's are both 5/8" so the 1/2" tubing I attached to it fits with no problem. The Chevy heater core is less expensive, about $23.00, but it has one 5/8" inlet and one 3/4". Other guys deal with it OK, but I wanted a neat and clean conversion to the 1/2" copper pipe.
These cores are all copper/brass not aluminum. This one was too big to fit inside my case, so I put it out back. I didn't care if it is not all inside.
HTH
Jim