is it loosing coolant(boiling over) when it runs at the high temp?,if not then the system is doing all it can do,and you need a different rad ,the aluminum jobs seem to be all the rave right now,and if you look around you can find one reasonablly priced
now a story from my history...............
way back in 69 or so i went on a trip from kansas thru oklahoma to texas,new mexico,arizona,colorado,and back to kansas,car was a 61 buick wagon,7 people,loaded to the gills with camping gear,even had a extra rack built on top in front of the luggage rack,car weighed in at around 7000# with the people in it,had ac thank god!,once we hit texas,it ran hot on the gauge,i tried about everything i could think of,different stat,no stat,different fan blade,antifreeze,even had the rad rodded in new mexico somewhere,nothing changed,rad was not plugged!,thru all of this it never boiled over ,just ran hot,and it was during the heat of summer,on the way home we went thru wolf creek pass in colorado,as soon as we started downhill on the east side the temp dropped to normal,and stayed that way till we were beating it hard across western kansas,and thru all of this it never lost any water,dont remember what the cap # was but i tried several different ones,,got home and unloaded and it never ran hot again,bottom line here is the cooling system was doing all it could,a bigger rad would have helped as it was only a two row,very similar in core size to yours,so based on that and decades of rad repair,im saying a bigger rad is in order if you want it to run cooler,but if its not loosing coolant its doing all it can,you might try backing off the timingto say four degrees and try it as is,also im sure you are aware that a new tight engine will produce more heat than a older looser engine