I have a 1966 Mustang with a 302 '68 V8 engine and it overheats. The car has a high volume Edelbrock Aluminum pump, with a double rolling timing chain, 4 row aluminum radiator from summit, electric fan 16 pull. The car was sitting for a few years before I bought it. It has some aftermarket parts for example Unilite Mallory distributor, a Holley 4barrel carburator. I took the water pump off and it seems to be good but I'm not sure, my friend the mechanic says it's good because it's pumping water. So then I put it back on and I took the radiator out and flushed it with water and I put it back and the overheating continued. you drive it like 20 mins and it starts to overheat. Then I took the heads and took them to a machine shop to be inspected and they were fine so I put them back on with new gaskets everywhere and they were correctly installed. Seems like the engine is really clean in the inside, like somebody worked in it before, which I like it. It has Shelby springs in the valves. Took the radiator out again because the top hose was getting stiff like before then I took it to a radiator shop and they flushed it, put it back on and refilled the radiator with anti-freeze and it went down better. I saw the difference in the temperature, I took it for a drive a few times, short trips and when I got back home the temperature was at like 3/4 between the cold and hot. When it cooled off I took the top off the radiator and there were residuals (like tiny rocks/sand) like before I flushed it, then I disconnected the hosed and flushed it again with the water hose, put it back on and it was working at some point satisfactory and after a few short trips it started to overheat again. Lastly I took it to Jiffy Lube and they flushed it, I thought it was going to work but it made it worse, like in the beginning.
Any advise on how I can fix this? Do I need a new radiator because it didn't move for so long, or a mechanic fan versus electric fan? Or because the top hose is getting so stiff, do I need a new water pump even though my mechanic says it's good?