What are the size of the transmisson pan.

Easier to just take a bolt to the auto parts store. They will be able to match it up or they'll have a gauge to measure it. The heads are usually 10 mm but there's no listing of the thread diameter and pitch.

Instead of using a Heli-Coil, you can just re-tap the hole and use a larger diameter bolt. Be sure there's no remaining RTV gasket sealer stuck up inside the bolt holes. If there is and the bolt butts up against it, that will give a false sensation of being tight.