I just went on a 150 mile trip in my 79 and got 11 mpg. Pretty good for a 460 C6 and 33 inch tires. I stayed on the highway with a max 55 mph speed limit and ran the factory AC most of the way. Only issue is the blower motor started squealing. Ugh.

In 25 years of owner ship this will be the 3rd time replacing the motor. I don't have my records in front of me but it has probably been 10 years since I put in a heater core and I am sure it was from a parts store-in other words not the best fitting. I never have been able to get full hot heat like it used to in the old days. I think it makes sense due to the difficulty of doing the fan motor to get a new core in place at the same time.
I remembered past discussions about 65 to 68 mustangs cores being reproduced in copper/brass that fit the factory AC trucks 73-79. see here... https://www.ford-trucks.com/forums/1214382-heater-core-rubber.html
Looks like National parts depot still has the cores. I noticed they also sell a top and bottom rubber cap. I realize this might need sculpting to fit but thought I might give it a try. The parts diagram shows the rubber seal going all the way around.


https://www.npdlink.com/product/heater-core-end-cap-seals/103093?year=1979
https://www.npdlink.com/product/heater-core-copper-brass/146337?year=1979

Does anybody have a picture of the original rubber? Also is VDO any good for a motor? It is difficult to find decent quality anymore.

I have a parts truck and a spare ac housing I might dig into and see if I can find and post one myself.

 

All I saved from ND. Laurel Mountain Mustang in the US has been closed for a while and the current web site shows contact info is Kuala Lumpur, Malaysia.
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"For anyone looking for a brass/copper core Advanced Auto part number 398304 is the correct one. It is also the same core that fits early, 68'ish mustangs.
Laurel Mountain Mustang is where I bought mine from.
Part number just in case it is needed in the future:
For A/C 18476-1A
For non A/C 18476-2
www.laurelmountainmustang.com/products/1973-79-bronco-heater-core-without-a-slash-c-brass

Also NAPA but makes sure and ask are they aluminum or copper? Copper is what you want.
Heater core, high output heater C6OZ-18476-B
?Antique Auto Supply, Arlington, TX (817) 275=2381
?Wesley Obsolete Parts, Liberty, KY (606) 787-5293

Dennis Carpenter, Concord, NC also has 1 but they have it listed under C60Z-18476-B, the third digit is "zero" instead of the correct letter "O". (704) 786-8139

Additional C6OZ-18476-B heater core applications: 1966/70 Falcon w/o A/C / 1966/72 Fairlane/Torino w/o A/C / 1971/73 Mustang/Cougar w/o A/C

Heater core w/ factory A/C or hi-low comfort vent system D3TZ-18476-A
Std Heater core D3TZ-18476-C
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Have you tried C&G for seals ect? https://cgfordparts.com/collections/...2F165747130505

Advanced Auto part # pops this https://shop.advanceautoparts.com/p/...chTerm=398304+