so are we to presume you bought the triple pass copper brass rad and it didnt help?,i once had a 65 chevy impala ss ,327 4 on the floor,i had ac added,always ran hot with the ac on,not boiling over but close to the hot mark on the gauge,one time i was toolin down the road in oklahoma,looked down at the gauge and it was runnin cooler than it had been,then the ac started blowin hot,turns out the belt had come off of the compresser,that story dont help you much ,but just thought id share it!
one time some yrs ago i had a guy come by with a henry j that he had stuffed a chevy engine into,seems as tho he might have used the chevy rad even,and it was runnin hot,as it turned out there was not enough room under the hood for air to flow thru and get blown out the bottom/sides,he took the hood off and it ran cool,just for the hell of it you might try takin the hood off and give it a test,lots easier than goin on a witch hunt by tearin down the engine
of course if you havent bought the new rad you might consider the a aluminum one,all the data says they do cool better,however,when chevy came out with the new body style in 88 ontheir pickups,most would run hot with the ac on,gm claimed there wasnt a problem,(no surprise there),i built some 4 row copper/brass rads for themand they cooled ok,after a couple of yrs they came out standard with a much bigger aluminum rad,and that took care of that,and i had a guy with a ford pickup that had a bad alum rad,i put in a c/b rad,it always ran hopt,even tho the c/b was thicker than the aluminum had been,
just a thought,what are you useing for a temp gauge???,might try a mechanical gauge just to check the electric and see if its correct