i've seen cores so plugged that they flowed only a trikle and caused fast overheating.
if your rad shop can't fix it.. see what they can source as replacement core.. get a hd core with more tubes.. ot less. I have seen some light dutry core with few tubes.. they are not as good..
The more hp you use.. the more cooling you need.
I have a 950 mutt/parts tractor with a non oem radiator from smal weird import car that I found to fit in.. it is less capacity.. I can make it boil if I work the tractor to full potential. for now I hung a carb on it for a 134ci eng to reduce the ponies and cooling needed. now I can run it to the hilt that the carb will allow and not heat... one day I'll find a good rad for it an put it's larger carb back on...