Coolant drain on block on 250/4.1?

[C10wip]

The coolant in my newly acquired truck looks like mud, the water pump is shot, the radiator is plugged and the heater doesn't heat, so I'm replacing the radiator, heater core, water pump, thermostat, hoses etc. I drained the radiator and took the lower hose off but only got about 6 - 7 quarts of "coolant" out. I'm going to flush it but want to drain all of the the flush water as well.
My question - on a 1980 250 L6, is there a coolant drain on the block, or will all of the coolant dump out when I take the water pump off? I have the shop manual but it does not seem to show a plug.

Thanks.