overheat only over 3000 rpm

Most, perhaps all, of what needs to be said regarding your cooling system, has been said.
So I just want to touch on ignition timing.

Most of us here have been tuning cars for over 50 years.
IDK about the rest of the guys but as for me, tuning the idle-timing by ear, for highest idle, is IMHO, a bad idea; unless you mean,
setting the idle-timing to 13* and then twiddling the Mixture screws for highest idle @13*; which IMO is still wrong,
because;
Even then, at 13*, the throttles are very nearly closed, which means the Transfer slots are also nearly closed, which means you are gonna have to crank the mixture screws open, for it to get enough fuel to idle ok. But then, as soon as you open the throttles, and the transfers come back on line, the AFR goes rich because of the fat mixture screw setting..

But if you have tuned your idle by advancing the timing for highest idle-rpm, that is just wrong;
because;
If 13* is the factory idle timing,
and 35* was the factory power-timing, at say 3400;
that means there is 35 less 13 =22* in the factory distributor.
If you are timing by ear for highest idle rpm, don't be surprised to see 25 to 30 degrees by timing light. Adding 22*distributor, to say 27*Idle-timing, is 49* of power-timing
which is WELL INTO detonation under Power.
And your VA could be say 15*. adding the VA to the cruise timing at 3000, could put you as high as 49+15= 64, but more likely is ~54* , if the springs are still working.

IDK anything about the Poly chambers, so IDK if 54*@3000 is acceptable cruise-timing;
But Ima thinking that if you are cruising at 3000 rpm, you will want the cruise-timing to be at least 48*.
But one thing I do know is that 49* of Power-timing in an LA will kill it.

Now, I have to tell you a couple of things;
1) the first time your engine cares about Power-timing is at Stall-speed.
2) the Second time it cares about Power-Timing, is from about 3500 rpm and beyond. These two data points are used to build your Power-Timing curve.
3) Part Throttle Timing is the thing that you want to concentrate on AFTER the Power-Timing has been finalized. This is done by modifying the VA.
4) Idle-timing is a big ol' window from about 5 to 20 degrees, and where you are in it, is nearly meaningless.
5) my current cruise rpm is 65=2240 in overdrive. But this method, described below, also worked for me when it was 65=2870
6) cruise-timing on a CityCar, can be sacrificed.
7) if your vehicle does not have a PCV system on it, but the carb has a PCV port on it, then the carb will never meter fuel properly except at WOT; so put a PCV system on it.
8) if your vehicle does have a PCV on it, with a carb that was never set up for it, then it will never meter fuel correctly at all; so get a carb that will.

Here's how I set my car up for cruising;
1) I revved the engine up to cruising rpm, and blocked it there.
2) without regard to the actual numbers, I advanced the timing for highest rpm at that blocked throttle.
3) I reduced the idle speed back down to cruise rpm, and fine tuned the timing again. Thus the engine is set up for best cruise-timing, at a no-load setting.
4) with the engine still running at cruise rpm, I now set the Mixture screws for best LEAN AFR.
5) now I read the timing. From that, I subtract 3* for load compensation, and then build my timing curve to hit that mark.
6) I return the engine to idle, and let her cool off.

Jus trying to help