383 overheating

Ughh, my buddy Gregg's '70 Chally. A car he sold 20 years ago and found again last summer..Undergone a Complete restoration, fresh motor thats making 502hp at the crank. Car has been back on the road for just a few short weeks again. A few details: factory style 26" rad., Block was bored .060 over and machined/balanced by a reputable engine builder. Compression ratio is just UNDER 10.0:1, no pinging or anything like that. Matter of fact just had a chassis dyno tune and picked up almost 50 more HP. Motor truly seems to run great. Gregg had noticed the engine temp stays cool at idle, even if it idles for some time. {Clutch fan works perfect.} Take it for a ride and the engine temp will rise sharply , pinning the temp gauge. My brother had found last Friday the engine had trapped an air pocket, and he flushed the system and wound up adding nearly a gallon and 1/2 additional fluid. Also he changed to a heavy duty water pump and a high flow thermostat for him. Again restarted the car, idled for an hour and shut it down to cool without overheating. Restart and idle/drive around the lot a bit and no more overheating. Then today Gregg again takes the car out for a ride. All seems Ok for a while and then the engine temp rises really fast. While the car overheating, there is no smell of antifreeze, and no plume of smoke from the exhaust. Also, there is NO sign of water on the oil dipstick and no film of oil in the radiator fluids. The engine builder who is VERY particular with his work has extreme doubts its the head gaskets. My brother & I both tend to agree with him as well.
I have a gut feeling the problem may lie with the radiator. Its @ 22 or 23 years " new". When it was new, sure a 3 core /cross flow was the bom but the car sat for 19 years and accumulated a total of 62 miles until gregg repurchased it. It hasnt been flow checked, I feel its not an efficient 3 core radiator and may be clogged with all kinds of ****. I bet the innerds of the radiator are pretty f'd up by now. Now hes got a healthy 383 with much thinner water jackets and when you stress the cooling system while driving the radiator just cant handle it.
I'm open for suggestions to pass along to him. My brother & I want Gregg to really explore every option before yanking the motor and handing this to the engine builder.