63 Dart overheating issue

When filling wait for the gauge to tick upwards to show the thermostat came open, and not installed backwards. Then top off. Your system wasn't originally designed for a overflow reservoir. You may need to go to a different pressure rated cap.

I am sure you have E10 in Canada. Ethanol will yellow and crack that filter. Go to a metal filter. Don't use one of those turd glass filters either.

I swapped an F body radiator into my Duster. It kept puking coolant. It was designed to have an overflow bottle with that radiator cap. I added one before going to the track.