The D15B8 engine is a peice of crap, it will never run "hot" in a normal condition even with the tiny stock radiator. Sorry to be so straight forward, but it's true.

1) Bleed the coolant system.
2) Check and replace the thermostat.
3) Check and replace the waterpump (if needed)
4) Check and replace the radiator (if needed)
5) Check and replace the cooling fan and electronics (if needed)

After that, you will not run hot.