1976 Dart Overheating and other issues

Good job.

Gaskets generally don't get sealant.
Sometimes a dressing is used to help hold in place during assembly. Indian Head Shellac, Hi-tack, and similar named products.
However...
If the sealing surfaces are badly corroded, then gasket can't conform. So then it may be neccessary to use a form a gasket or similar RTV.
Hopefully you don't have to go there.

Tip: With copper radiators and relatively open coolant systems (the recovery tank is still open to atmosphere), the best coolant is 'conventional' green. Theadvantages are that the corrosion inhibitors in the 'conventional' green work right away, and they protect solder and copper reasonably well. Universal coolants, regardless of color, are hybrid organic anticorrosion technology. They require time to build up and become effective, and even then are not usually great in protecting solder an copper.