Small Block 305 Bottom end clacking (sounds like small diesel engine)

[81GMCSierra1500]

UPDATE:

Running fine, took it into a shop to take a peek, they said the overheat issue was because of a bad thermostat (no idea how old the one in it now is), and I had just put a full tank of not so great fuel into it, so the hot running temp, low-ish oil level and ****ty fuel all combined to create this nasty situation. I will be installing the new thermostat with some fresh daylight tomorrow, and see what happens...