383 overheating

Pull the rad and either flush it yourself to see how it flows or bring it to a rad shop and have it rodded out. The quick way that rad shops check em is to block the lower bung with their hand while filling it thru the top bung and when it's completely full releasing the lower one to see how fast it can empty. If it doesn't dump out in a few seconds then it's plugged. You can try and back flush it but it's only temporary for the most part. If that's the case then you can have it rodded out or recored/replaced/upgraded.