Need an ID on cooling system issues

Newer engines are set up to run hotter for emissions reasons. If you drop a cooler thermostat, the computer will adjust to those temperatures but not for the better. My ISX fan comes on around 205/210 just to give an idea.

 

Yup normal operateing temp for my MX13 was 180 coolant/200 oil fan would come on around 215 and oil at around 245

My isx runs around 175ish and fan kicks in at 205ish oil runs 200 and fan kicks in around 250.

If your worried about temps just flip on the fan switch before you pull a hill and eyeball the temps. If they begin to climb above those fan kick in temps. Drop a gear or two. You will be fine.

 

I vote for 5-10 degree cooler thermostat than stock in summer & stock thermostat in winter. If you use a cooler thermostat in winter, newer emissions engines really won't like that as your coolant will be below the minimum normal operating temp. The coldest coolant you can have that makes the emission systems happy is what I'd go for. No single answer if correct, as every truck's aerodynamics & radiator setups are different.
As far as running hotter now, your new aftermarket radiator might not be as good at transferring heat as your previous one, there might be air in the system now where there shouldn't be air, or if you took a while to change everything out, rust/buildup might be in the system which would reduce heat transfer in the radiator and engine, final thing might be correct antifreeze ratio (use a refractometer to verify).