Gene, all comments made by others and you are completely correct. What is not known is how much crud is in the block.

I recently worked on a 235 that had been well taken care of for its entire life. When I flushed the system while the engine was still in the truck, the back plug was clogged. I did a little tinkering with a coat hanger and it began to flow freely almost immediately without any more black stuff in the mix as I finished the flushing.

If your engine was maintained throughout its life and flushed regularly including pulling the back plug, you might not have the deep crud build-up that others have experienced. You might be lucky!

As others have done, I changed out the back plug to a petcock to make draining easier in the future.

I use a shade tree mechanic's approach when flushing a block. You want to avoid getting any of the crud into the radiator or the heater core. Remove the radiator hoses and the heater hoses at the block/water pump. Remove the back plug. Pull the heater hose fittings from the thermostat housing and the water pump and plug with threaded plugs. Remove the thermostat. Insert your garden hose into the upper radiator hose fitting and pack it tight with a wet rag. (Keep this packed tight during the process to keep pressure in the system.) Turn your garden hose on full blast. Turn it on and off to pulse it. Occasionally kink the hose to let it build up pressure and then unkink it to surge water through the system. After doing this for a while, plug the lower radiator hose connection at the water pump with an expansion plug (a couple bucks from the plumbing section at Home Depot). Repeat the process with just the back plug removed. Then pull the expansion plug and let the system drain. Turn the engine over to let the water pump spit out any crud in the impeller before reattaching the heater hose and the lower radiator hose.

It is easier to do this with two people...one handling the hose and one keeping the hose in the thermostat housing with the rag packed tight.

A deep clean by pulling the freeze plugs would certainly be more thorough.

Good luck,

Matt