GM-GMC: my 1989 chev305, rad cap, coolant

Question
my rad was pressure tested all is fine but when it starts up it almost over heats then all is fine I replaced my thermostat 3 times now running with out one..the rad gets so hot it almost gets to steaming..help no air block either did a complete flush

Answer
Try replacing your rad cap.  If it doesn't hold pressure than your system will boil and cause an over heat. It needs to hold pressure to modify the boiling point of your coolant.  That is the only thing I can think of other than a rad blockage.
Hope this helps