It may be the one on the power steering cooler? I have a 1996 5.7 and mine has 6 plus on it. There is 1 plastic blue plug on each side of exhaust elbow. It has 1 brass pipe plug on each side of the block and has 2 small brass pipe plugs on the back side of the risers. My neighbor has a 1999 5.0 and he has pretty much the same setup though his block plugs are also blue plastic and he has an extra blue plastic one down on the power steering cooler.
Just make sure you remove them all and actually get something out of them or else they are plugged. If they are plugged, get some wire and push it into the hole to clean it out.
Disconnect all hoses from the thermostat housing as well and dump them too (lower them as low as possible). Then start filling them with pink stuff until it starts flowing out.
Start with large hose to water pump. Fill it until it comes out good and pink from block plugs. Insert plugs and continue to fill until full (I normally remove thermo housing and replace thermo and gasket so I just fill till level with opening). Next start filling 2 smaller hoses (these go the exhaust) until it comes out the lower plugs. Put them in and keep filling until it comes out the higher back plugs. Insert them and then keep filling until you see it come out the back of the boat (help to have someone on the outside looking). Next start filling the last hose (small hose off of right side) until it comes out the power steering drain (if you have one). Insert plug and keep filling until it comes out the back of boat again.
I just did mine last weekend and it took about 5 gallons to do it this way. Just make sure you keep pouring for a bit after you see it come out to ensure you have pushed the water out too. Normally you can see the color change from light pink to dark pink.