FLYwheel ring gear

[Bluebell]

How I was taught was to heat it gently with the gas until a wooden match rubbed on the surface charred and slipped easily. No hotter or you will soften the teeth and they wont last long.
The freezer and oven sound good to me. Be organized. I have heard of guys getting it wrong (wrong way up) in the heat of the moment. Then the new gear is scrap.
Have things clean and all burrs removed. You only get one shot at it.