Cleaning up the firewall????

[1971c10La]

Need little help here trying to clean up my firewall I like the clean look but there's 1000s of holes in it well maybe less than 50 but still I don't wanna weld up a hole that don't need to be welded by the way I'm plan on going with a vintage air kit so I know I got get some sheet metal to cover the three big holes what gauge metal should I get? Here are some pics I know the PO had screwed some holes in it so not sure what's stays or goes any help very much appreciated thanks