Looking for some bodywork advice for to the best way to shave part of a firewall (for this e body). I will be installing a vintage air system so i do not need all the holes for the old plumbing.

Would i be best off welding a "patch" sheet over the area marked in red and using filler to blend it in?
Or put the patch on the inside of the firewall and weld the holes flush to it then filler holes.
Or cut out and weld flush pieces for each hole individually?

thanks