63 Dart overheating issue

Mattax said
Lets start with this. The inability to hold pressure anywhere in the system will allow the temperature to rise once vapor begins to form in the system.
See if you can get your hands on a tester. They usually have adapters to check caps as well as the system.
Looks like this
View attachment 1716113661
Side note: A radiator without a recovery tank will be full cold about an inch below the cap.

With Regards to timing:
A. Its seems close enough that should not be the cause of overheating.
B. RPM RPM RPM
The RPM of the measurement is important. Centrifical advance adds to the intial above 600 or 650 rpm.
So 2.5* BTDC at fast idle is retarded.
2.5*BTDC should be at slow idle specification. I'll guess around 600 rpm is the spec. Could even be 550 rpm "with lights on" (so the alternator is adding load).

No vacuum advance at idle is correct.

I think you misunderstood what I was saying, or I miswrote. Either way. It is holding good pressure. It only pukes out past the cap once it almost reaches hot on the gauge inside the car.

RPM was set close to 600RPM (nice slow idle). That is how the timing was set, with vacuum disconnected and plugged.

Fluid level was set to about 1 inch below cap as well (no overflow bottle...yet).

All great information. Most of this I am familiar with, some not as much.

I am pretty confident I have found the issue to be a plugged radiator. Reason I didn't go there immediately was the fact that it was redone by a reputable shop. Once I got it out and drained and got the scope inside it is clear there are a bunch of clogged tubes. Must have had a pocket of crap that didn't loosen up with the original air blow out or original flush. Thinking it had to come loose the first time the car got driving and heated up and bouncing around. I was just remembering that I am pretty sure the first few runs it only ever got up to the low end of the operating range, where I figured it should be. Either way, it is dropped off at the shop to be hot tanked and flushed and flow checked to be sure it's good to go.

I also forward and backward flushed the engine after the rad was pulled to try to get any residual dirt out. Hopefully I'll be good to go for some time to come.