Pull the block plugs while flushing, and flush with simple gree 1/2 gallon at a time, probably 4 times. Block plugs are 8mm allens, drivers side you can get at, passenger you gotta drop the starter.
To flush, remove t stat, put in simple green, top off with water, run at 1200 rpm for 10-15 min after at operating temp. Then pull 1 block plug, put hise into heater hose on passenger side of motor (under alternator) and flush for 10-15 min. Re install block plug and pull the other one. Repeat procedure with both gallons of simple green. If still not completely clean, get more simple green. Once oil residue stops coming out, then flush using same procedure, but with fords vc9 to remove iron deposits from block. After complete, replace t stat and probably degas due to baffling trapping oil. You may need to repeat procedure at next oil change as well depending on how bad it was.