Yep that plug has to be removed, on the Vortec models, pre vortecs didn't need that done. If you want to use AF make sure it will protect to the lowest temp you will have.
The way to put af in the engine and manifolds:
Disconnect bottom end of big hose from thermo housing to circulation pump, that drains water that the block drains don't drain. Then, reconnect it and disconnect it at the top end. Now fill that hose till the AF spurts out of the thermo housing neck, reconnect hose, engine is full. No need to remove thermostat. Now after draining the exhaust manifolds, disconnect the hoses feeding them at the thermostat housing. Fill each one with AF till it runs out the exhaust housing on the transom mount. Reconnect hoses. Disconnect the hose going from the transom mount to your raw water pump (impeller) at the impeller housing. Hold it down to drain water out. Then put the outdrive down. Now fill that hose till the AF runs out the outdrive water intakes. Re connect hose to impeller housing. Finally, disconnect the hose from the impeller housing to the thermostat housing at the thermo housing and fill it with AF as well, then reconnect.
if you get down to close to zero, use -100 not -50 or -60, they get hard, at roughly 7--11 degrees above zero.
