No problem Scott, helping others is part of why we are here.
Regarding getting the heater core, I usually just order mine through Ebay. Here is a link to the one you need:
69 70 71 72 73 Dodge Plymouth Chrysler A/C Dual Heater Core Coil Heat HC5777 NEW | eBay
If you need a condenser as well, then search for 1972 Imperial condenser and one from ACGlobal will come up too.
For the Auto Temp II upper dash push button control unit, the part number is 3431019 and is available from Deception Pass Motor Parts in Anacortes, WA:
Search Mopar Inventory: Len Dawson's Deception Pass Motor Parts - NOS Mopar
He shows he has one left in stock and Len is meticulous about keeping his inventory current and is very good to deal with and has good prices relative to other suppliers. Arizona Parts is also a good supplier but Marty there is showing he has none of those left.
Those rarely go bad in my experience so you might want to make the other changes and try out the system before actually ordering one or you may want to have a spare on hand for the future.
Also, for removal of the heater core, besides taking out the right front seat, you will also need to take out the right kick panel and the door sill plate and do your best to get the carpet and carpet pad out of the way. That will give you much better access to the lower housing screws which are the most difficult ones to access for me at least (as best I can recall since the last one I did was several years ago now). You will also need to remove the glove box and a couple of the a/c ducts that go behind that area. Those ducts are very securely attached so pry gently side to side to get them off the center distribution vent and the lower right vent/outlet. So be careful with removing those ducts since if you don't do so gently side to side, you may well end up breaking one of those a/c outlets.
Keep in touch as you proceed as necessary.