The
TWOA newsletter, Issue 25, December 2013 (Winter Edition) has an article titled "Rebuilding the AC / Heater unit... Brent takes us through a complete rebuild" by Brent Roth.
In this write-up he didn't replace either the heater or the cold matrix but he might have suggestions for substitutes.
I'm hoping that I find both matrices in serviceable condition when I get to this job. Each matrix looks unlike equiv parts from other cars I've seen.