Do you have a mechanical fan at all? perhaps clutch failure?
what was your driving condition at the time?
Maybe your coolant bottlecap has failed, allowing boiling to occur.

I know the intercooler should take some cooling load off the radiator, but it is my feeling that the stock diesel radiator is somewhat marginal (although many 5.0 Ford motors seem ok with it), and anything less could be insufficient. Possibly the issue could be coolant capacity rather than surface area, I bet the D24 hods more coolant than a redblock.
Looking to Ford for inspiration (maybe not the worlds best plan) compare the radiator on a 7.3l non turbo diesel to the one in a 7.5l NA gas truck... the diesel has a special core suport to hold the nearly twice as big, equipped with lift hooks becaus it is so heavy radiator.

Maybe it is just the age of the cars, but every Volvo diesel I've driven starts to heat up(passes half on the guage) on long extended hillclimbs at full power. Fords and Benzes dont(usualy slower though). My time to overheat on hills is pretty variable, and I'm not sure why. My car (with two different engines) has alwyas been prevented from criusing over 95 MPH by heat buildup(PO confirms this). Not so with the SDL mercedes(heavier car with bigger aluminum radiator), which accelerates past 90 going uphill without even wanting too (D24 has trained me to not let off, a bad habbit in faster cars).

The more I think about it, the more I think loss of pressure in your cooling system.