Coolant at -40 F with a 15 lb preasure cap will hold 243F temperature.
Any time you have an overheating issue, the first thing you want to check is the rad cap. Also make sure the cap you get is good, the little flat valve when you push up on the main rubber must be tight.
There are lots of them out there that are junk.
If the neck on the rad is bad a rad shop can repair and or replace that. While it is at the shop get the rad cleaned, it is cheap, and tested. They will paint it with a tar, then put on a 3/16 inch, metal bug screen.
When you refill it take it to -40F it cools better.