Were you riding deep sand or mud?
Lots of hill climbs?
How long have you had the 240 kit in?
Any chance your jetting is a little too lean?
Are you running 50/50 water and anti-freeze?
An especially brutal trail and a hot or humid day can really cause the overheating, so it might just be a one time thing.
For example, I had a '99 CRE 250 Honda for several years, and that thing never used a drop of coolant. Except on one ride where I lost about 3 pints in 10 miles of hell.
High humidity can hurt the radiator's ability to transfer heat, but temp of only 75 is pretty moderate.
FWIW, lugging the motor and riding at low rpm will generate more combustion heat than revving it out. I had a 240 kit in my old '84 (air cooled) and that thing was like a 500 off idle.
Basically, if you are riding a gear high and chugging, that can contribute to the overheating issues. A lower % of anti-freeze in the mix might help transfer the heat a little better.