Another overheating situation 63 Slant six

Timing has been set at 8BTDC to 0TDC to retarded so much it lacks any power at all and still overheats.
The radiator is supposed to be a stock brass/copper I assume, not aluminum, radiator from a newer dart between 67-72 as I remember, it has been a few years since I bought it. Overall it fits the opening but is thinner than the original one.
I bought the radiator off craigslist still boxed but it was rebuild for sure. Ill check the row count and fire her up today to see if I can better document the heating situation.
Cheers and happy spring.
When the engine is cold, take the radiator cap off and start the engine. As the engine thermostat opens, do you see a rush of water into the radiator? After the rush, do you see good movement in the radiator, or is it almost imperceptible?