Overheating and vapor locking

4 core radiators are meant for very high horse power vehicles. Meaning in excess of 1000 hp. The idea is there are more rows of coils so that the coolant will have longer in the radiator and come out colder so that it can cool an engine that is able to produce that much hp. I would not think this is the issue. However, it is too much radiator for this vehicle.

However, you premise is basically what I am thinking your issue is. Running at that high RPM the coolant isn't getting cold enough so an auxiliary fan on the front of the radiator that pushes more air when the engine starts to overheat would solve it.

Before you do that, here is what I would suggest, get an infrared thermometer and when the engine starts to over heat, pull off the road ASAP and start measuring around the engine and cooling hoses to see what the temp is. It should be within 5 degrees all over the engine except for the lower hose. This should be much cooler since this is just coming out of the radiator. If it is not, about 10 degrees or more then the radiator is not cooling it enough.

You can't really make a wrong choice with these but here is a link to a pretty common product. Clearly, I don't endorse this company but there are pretty popular.

Here is a video from them that I think answers your questions. However, this is to be used in addition to your existing system. Let me know if you have more questions. Thanks

https://youtu.be/7QO5zHJmtYY

Very good possibility unless you have a heater core bypass. I don't show one but if you have one, then if it gets clogged it routes the coolant back to the system so that it doesn't cause this issue. Check your inlet hose to the heater core and see if there is a "T" on it that goes to the outlet. If there is then you have a bypass and you are correct that it is probably clogged which is why you are not getting heat. However, it is just routing the coolant back to the engine so it is unlikely that it is causing your issue.

If you don't have one, I would suggest testing this by creating one. Just disconnect the inlet and outlet hose and get a connection piece and put them together so that it bypasses the heater core and then drive it.

Let me know what happens. Thanks and good thinking trying the heat. I never would have thought of that with it being summer an all. Thanks