Be aware that welding the head might permanently soften the aluminum. If the combustion chamber was welded near the seats, the interference fit of the seat inserts might loose it's press and could possibly fall right out of the head when engine reaches temp for the first time. Then you'll be right where you started again..
A soft head can even make it so that your head bolts won't torque up because the head just squishes as you tighten the fastener. You could fail head gaskets too.
I would definitely seek the advise of an experienced engine builder who can test the hardness of the head b/c you might be working with a head that should be deemed srcap.
A new B25, V25B, and W25 Subie head should have a hardness of around 92 on the Rockwell "E" scale. I wouldn't run anything below 80.