well bleed the coolant first. leave the radiator cap off and run the car to let all the bubbles out. once it stops bubbling then put the cap back on and fill the reservoir up to the max marker. try going for a drive again and see if it's happening again.

when i had my b16a swap in my EH, the old radiator that was on there was just messed up. all the heatsink finds were all bent up and there was some rust all over it. the OEM single core radiator should work fine even with a B18C5, but old radiators that are rusting will not transfer heat to the air as efficiently. seeing as how these radiators are single core, any changes in airflow across the fins is going to affect the temp especially on a bad radiator. if the coolant has been bled and it's still heating up, you may need a new radiator (thats of course if you've already confirmed that the thermostat is good and the coolant temp sensor is working).

anyways to summarize:

1. air bubbles in coolant?
2. thermostat opening?
3. temp sensor good?
4. rusted/damaged/leaking radiator?
5. old water pump?

it's probably not the headgasket unless your unable to bleed the air bubbles out of the coolant forever... good luck!