Engine plug removal - short block swap

I need some advise of removing the engine plug, (boss) near the front under the manifold. It a square socket but smaller than 3/8 and larger than 1/4. Do they make a 5/16? or do you have to use Metric? I tried grinding a 3/8 extension to fit, but it softened the metal to much, even after using heat on the plug.

I'm swaping a short block from 91 into a 87. The head and manifolds are done wireing stays the same, Pilot needs to be removed for the torque converter (AW4 Transmission) to fit, other sensors are switched, this is all I have left, please help if you can. I need to get the plow truck fixed before snow flies.

Oh Yeah -it's a 4.0

Mixing a little aceton with your penetrating oil helps it leech into the rust or sealer. Tapping it with a small hammer, will help the oil penetrate. 8 MM or 5/16 is used sometimes in plumbing fixtures and refrigeration applications. May have a socket adaptor at a plumbing supply place.