I put a GDI 3 core in the '88 XJ I used to have 3 years ago, the guy that has it now still never has any problems with temps.
I put a GDI 3 core in my current '88 MJ 2 years ago and it still works pretty good.
I don't have any personal complaints with them, but GSequoia is right that a lot of people here have had problems. I would look into the CSF radiator, a lot of people seem to be happy with that, and i think it's $150.