Moparmatt72: If you're going to go all out with a cage and an aluminum firewall then this is the perfect time to do it right and get some experience at the same time.
#1: Go to Lowes, Home Depot, Ace, etc. (I use speedymetals.com) and buy a 24" X 24" piece of 1/8" (.125") aluminum.
#2: If you don't already have one, go to any of the above and buy a saber saw (jig saw) and some blades to cut non-ferrous metal.
#3: Make cardboard patterns sized to cover the holes you want to cover.
#4: Transfer the patterns to the aluminum with a sharpie.
#5: Cut out the patterns.
#6: Hand file the edges smooth.
#7: Drill several 3/16" holes around the perimeter of the new plates.
#8: Using self tapping #8 X 1/2" Tek screws (same source as the aluminum), screw the block off plates to the firewall.
#9: Optionally, you can polish the aluminum with fine sandpaper and Mother's aluminum polish to shine like chrome.
#10: Optionally, adhere heat/sound mat inside firewall, although if you're going to ditch the firewall I wouldn't spend the money.
#11: Stand back and admire your work!