Another HELP question this time Urgent

[64Magoo]

I just rebuilt my Rochester 2gc and put it back on the engine. It ran great for about 5 minutes and then I started hearing a loud knocking sound coming from somewhere in my engine but I can't tell where. Next thing I notice is puddles of what looks like blackened coolant from my exhaust pipes about 4 feet long and sprayed on my tires (I have turndowns just after the cab angles out slightly towards the side of the truck). What happened? I plugged the intake manifold when the carb was off so I don't think anything got in it, and I noticed that my coolant is pretty much gone after this happened (it was full prior). My water pump is new so what could be the problem?