FYI- The SSR INPUT pins accept a dc voltage so if you use a PWM pin then you can control the duty cycle of the SSR , which I think
is more what you had in mind. You may want to map the temperature from the thermocouple chip to the analogWrite value you send to the PWM pin controlling the SSR. You'll have to throw some PID code in there somewhere but basically that is the answer to the question you were asking.