I would call of visit a local O'Reilly's and ask they look up a Murray #94503. I used a link higher up in the thread to get mine but the site appears to no longer be up or is at least temporarily down. Either should be a fit for your application.
Tip for you too, I had to slightly modify the heater box holes on mine because the in and out lines on the heater core were just barely too off to bend through the firewall. Simply beveled the holes to open em up a little more and it went right through.
I'll look around and see if I can find another source for a core and post it here for you if I can find anything.
