I'm not really looking for answers because I don't believe it can be fixed. There's nothing left to do. I just have to realize that I can't go over about 60 MPH. Below that and I can drive all day. Over that and it gets hotter and hotter until I shut it off.

I just had the engine rebuilt for the second time. This builder allowed for larger clearances around the bearings because he thought it was too tight. Doing that did resolve the problem of the engine being so hard to turn over, but not the over heating. I used an aftermarket intake manifold this time that doesn't have the heat cross-over built in. I rebuilt the 3 core radiator with new brass. I used a high volume water pump.

I haven't even reinstalled the AC compressor and I probably never will.

I had the transmission rebuilt twice this year and it still leaks. There's an endless number of problems I've failed to fix after working on this car for most of my life. I literally gave myself a hernia trying to fix the drum brakes because I thought the wheels wouldn't fix disc brakes. You can't buy all of the parts for the front drums, so it was a wasted 2 month effort that cost several hundred dollars. And it turns out that the drum wheels fit onto factory style disc set-ups just fine.

FYI - the car has always sat low in the back. I bought new leaf springs from Eaton that were supposed to have a 2" lift and they had zero effect. It doesn't look any different at all. That was another expensive mistake.

Swap the motor? Move on. Enjoy as is?

I’ve theorized that at highway speeds the airflow limits the air getting to the radiator. Mine runs hot on the highway. High rpm’s and not enough air to the radiator. It would be fun to put it in a wind tunnel

That's a beautiful '69 you have in a post.

It sounds like you don't want to revisit the overheating but I would like to mention a few things. I'm more familiar with my '68 400HO with AC but I can't think the '68 is much different.

1st, you mention a recore on the radiator being 3 cores. A 400 with AC should have received the most robust radiator from the factory. I get confused by radiator rows verses cores but I thought mine has a 4 core Harrison UP code.

Do you have the seven blade fan with the clutch that gives it a high and low speed above and below 150F? Is that fan about half way into the fan shroud +-?

Do you have all the three baffles and two seals that direct air flow into the radiator?

You don't want to discuss it. that's OK. It's just that my '68 with 400HO and AC has never overheated in 188k miles. (Other than one stuck thermostat.)

You're dead on, Jim!
I have a 400 with A/C and use the original 4 row radiator that was recored, all baffles in place, original fan blade and I run 180-190F all year long, even at 100F out there. What really helped mine years ago, I decided to replace my fan clutch with a HD for a bigger motor. You can hear the additional hissing sound, but it helped a ton keeping the car cool. Good luck, -pay attention to those details and providing your timing is good, you should be able to keep your car from overheating.

Thanks.
I have all of the baffles. What seals are you referring to?
Where did you find a 4 core radiator? I wouldn't have rec-cored the one I have but the guy who rebuilt the engine had that done and told me about it afterward.

If you have all the baffles, you have the most important of the parts that direct the air flow. The seals are a rubber-like (they have also been called masticated fillers) seals that are on on the right and left side in front of the radiator. See the bottom two illustrations on the attached diagram from the 1968 Service Manual for where the seals are on the '68. I am not sure if '69 is the same.

I had my original Harrison UP radiator recored and restored a few years ago. I had also successfully run an aftermarket from a classic car vendor for a few years too.

Others here with knowledge and experience with available radiators or, if you have a good radiator shop you can talk with, they may be able to mention vendors and discuss radiator technology a lot better than me. Radiators are not cheap so by consulting with an expert before "trying" a new one might give you the level of cooling improvement you can expect.

Some places advertise by the number of cores and other by the number of rows. Some have original equipment appearance and others have all kinds of designs. Some are copper and brass and some are aluminum. And, occasionally there are original radiators that pop up for sale.

The rubber seals are the same (or at least very similar) on a ´69 too.