I p0sted it a long time ago but I pulled this 1975 Chevy Luv heater core out of the scrap yard for 9$ along with a few others.
This is the one I am currently using:
It has a very similar fin pattern. The problem I have with it is it is hard to get air through it. The only way I could get it reasonable was to make a acrylic adapter like shown and have the air sucked through (note there is a big hole cut in the bottom of the case below the rad). Trying to push it through wasn't as effective. I got a 120mm Delta 125CFM fan on it now and not much air gets through it. Still seems to do all right though