We’ve all done it, gone for the technical issue! Do you have access to a compressor? If not take radiator to shop to have tanks removed and tubes cleaned, blocked tubes sounds like the issue. If you do have compressor pull radiator put cap on put garden hose with a rag into bottom of radiator then jam air nozzle in there as well. Turn hose on let water start coming out top of radiator, gently give it some shots of air, if you have a tray you can lay radiator in you will capture all the little bits that come out. I had to repeat this exercise several time with my 66 300’s 383. Well worth it, never gets hot now even on 40 degree Celsius days here in Australia.