Place your barrel onto the case and look at the bottom pieces that fit into the casings , use a small ruler for eg and you will see if everything lines up and is clear .
Open up the oil holes on your piston too to help with cooling the bridge .

I too just get stuck in there with an air grinder. But don't use grinding stones on aluminum as they'll get clogged double quick.

I have some big roughing cutters that can really chomp the material out, and then progressively less savage.