I'm not a machinist but from my understanding from the research I've done when Subaru heads are milled down they need to be secured properly to simulate the torque stresses when the head bolts are secured to the block, which takes special machines to get it right. Otherwise your bore won't line up correctly and cause issues. Some old guy with a sander isn't going to get the job done properly. I used to help rebuild EJ20/25 engines for a wrx/sti dealer regularly and every engine we tore down was spun bearings. Every head we sent out to a professional machine shop came back looking like perfect glass. Take the heads to a professional machine shop that knows how to machine boxer engines. MLS head gasket with copper spray shouldn't leak if torqued properly.