Reheating of a GM V8 350 Chevrolet

First of all I thank those who read this problem and much more if they give me the solution, this is um GM V8 350 according to my research The year is what I would not know but I think it is 85, a general repair was made a general repair PQ was already overheating before and we thought it was a problem of the packaging, also that time that the truck did not move, repair of water pump of water. Building the engine and 10 minutes to the top, it was thought that it was a problem with the rings, but the right light was given, from there we saw that its cylinder head had very small water ducts in diameter, I sent to prepare packaging with large holes, and I endure more than 1 hour but when it took to try again, and from there there is an issue that I have in doubt, when I take a question, when I take the cover, when The engine starts, it is seen that the water circulates, but when you stop accelerating, it is seen that the water goes out to presignment, and this is, I cannot give it a solution, the only thing that I have left is to get a radiator with vertical gutters and see if it helps to pass the water faster, I appreciate the solutions you provide. PDT: It has no thermostat and its drive is by Correa (belt)