'79, 4x4, standard cab, 300 I6, 4sp, non AC.

Will a high output heater core fit in place of a standard heater core? If it's not a direct replacement, how hard is it to make it fit? TIA.

 

Thats a good question, I didn't know there was a high output heater core and a standard one. I looked on a parts store website for repalcement parts. It does show both as being available but the weird thing is it has the same part number for both. If I was you I'd go to a parts store and have them pull both of them and see if there is a difference. If they look to be the same thing then I don't think it would matter which one you got. You might want to ask the parts store person if they know what the difference is.

 

At one time I didn't think there was a difference in the plenum heater assembly.
But I was told that because of the size of the HO core compared to the std core. you will need the correct plastic assembly.

There are tons of these in Canada

 

You know what is funny? A high output heater core is usually cheaper than the standard core!
The high output cores are physically larger. They are the same width as standard, but are taller. Because you can't really change the position the hose barbs come through the firewall, the extra height must extend downward.
To do this, you remove the heater plenum from under the dash. (don't forget the three screws near the center of the firewall, engine side). Your Haynes manual should explain this procedure. If not, let me know and I will be happy to supply you with instructions. The hardest parts is re-installing, but it is not that difficult. It is necessary in order to replace the core, so be patient and you will be well rewarded.

With the plenum out, drill out the aluminum rivets that hold the plenum halves together. Seperate the halves, and take a good look inside. There is a 'shelf' inside that supports the heater core, and there may be a rubber insulator between the core and the shelf. This shelf is what needs to be removed to accomodate the taller high output core. Don't worry, it is plastic, so a Dremel or a hacksaw blade will suffice. Just cut it flush to the floor of the plenum.
What I can't remember is if the high output core sits on the floor of the plenum, or if I built a new support shelf. It is easy to leave it or fab something up, like a scrap piece of angle aluminum.

I placed foam weatherstrip tape around one half of the plenum and placed the halves back together, installing new rivets through the holes. Reinstall the plenum, and be ready for the blast of heat! It is really worth it in cold climates.

It is much simpler to do than it sounds.
Good luck!

Mark