Generation IV External Engine - Can anyone tell me cylinder head material ?

I got a set of 5.3l 862 heads, I need to weld up an exhaust flange bolt hole (followed with drilling and taping it). Does any one know what type of aluminum gm used for the casting ? Thanks..

I'm gonna be using a tig. I'm just trying to figure out the BEST filler rod to use.

Why not just helicoil the hole?...should be WAY less work.

I think it's probably something along the lines of an A356, but I don't know that for sure.

The hole might have been so stripped out/destroyed TIG might be the only option.

The heads are sand cast of 356 aluminum, heat-treated to the T6 specification.