Update on Project Unicorn:
The aftermarket heater core I ordered from NAPA turned out to be the right box but wrong part in the box, told no parts in the napa system, so I got my money back. I've ordered aftermarket heater core from specter and used "clasps" from specter. I obtained new o-rings from Toyota. Today I took my OEM heater core down to radiator shop and will get it "Re-Cored". I plan to use the OEM heater core and install it once I get it re-cored and I get my used clasps=clamps. One of the clasps that came off my truck looks to be in worse shape than the other, and so I want to replace it now, as I suspect my intial problem was a failure at the o ring. My truck ran for some time with the heater core bypassed, so it a little bit of a challenge to pin-point the location now, as the heater core itself does not show obvious signs of a leak. Since we are talking about a 30+ year old part that is a pain in the @$$ to get to, I'm going to get it re-cored. Whatever other parts I get will be kept as spares.
Local radiator shop says (1) they can re-core the OEM heater core (2) they can solder the pipes to the heater core if needed without issue.
I plan to go back with re-cored OEM heater core, one good OEM clasp and one used one, new o-rings.
I plan to save the heater core from specter once I confirm it works , in case things go to $ell.
I also plan to stay with the OEM configuration unless I find I can't get it to work for some reason since it did last about 30 years or so. If it screws up, then I will have the pipes soldered to the heater core.
Parts will not fully show up to my place before next weekend I'm guessing. Re-Core of OEM heater core is looking to cost around $300.00 US as the radiator shop says they get charged a min amount on cores no matter the size.
Suggestions I would make. Keep up with your OEM heater core and have it re-cored if required. I suspect their might be alternative 0-rings out there, I just don't know where and what specific source. What my observation is.....you have to use a similar size o ring both is diameter and thickness or its not going to seal. The clasps or Clamps that mate the core to the heater pipe is unicorn status from what I've observed so far, used is the only source I'm aware of.
As things progress I'll update. I'll also make sure to update the status of the core and used clasps. once I get them. I expect I want get my re-cored heater core back from the radiator shop until late next week or longer. The radiator shop seemed to think no big deal in gettting re-cored or soldering the pipes if needed.
We'll see how things play out.