some good info in this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=690735
here is the main part:
OK I gave the local auto parts store another try. This time I decided to look under an alternate year. I noticed that there was a heater core for a "fresh air" heater and another option. "Re-circulation" or something like that. Under details of the heater core by Murray P#94691 the dimension of the core matched up and the picture looked the same. So I ordered that one. Well I got the core and installed it. It fit like a glove. no shimming, no bending or anything.
Lessons learned include:
always verify the guy behind the counter, unless you deal with vendors from this forum. They are pretty damn good at what they do.
ask for help and/or research this forum. There is lot of info on here.
Now we have two known part numbers for the suburban "fresh air" heater core
Murray Part # 94691 (O'reilys)
and
pro-source Part # 91691