You want to use polished stainless steel. I believe that it is 22 gauge and comes in a 3 foot by 8 foot sheet. One side is "mill finish", some call brushed chrome (like your refregerator is, and the other is "mill polished". In order to get it to gleam like chrome you are going to need to buff if with jewelers rouge and a buffing wheel.
Stainless steel is much harder then steel. You don't want to use a heavier gauge because of the difficulty in bending it.
When you cover the firewall, you have the option of using rigid foam insulation between the bulkhead and the stainless panel. I believe that I was able to get 1-1/2" thick foam in there.
The gas tank shielding can be let in place and just covered over with the stainless.
I believe that I had enough material left from one sheet to make new stainless splash shields shields in the fender wells with the left over materials.
I didn't make a a/c condenser shield from stainless but bought one from Hall Pantera.
All the stainless sheet on the car changes the nature of the basic car and is definitely a plus.
Keep in mind that it is likely that if you buy it from a vender that it isn't easy to ship the sheetmetal without it getting crimped from shipping. I made my own except for the a/c condenser cover.
I know. Will is going to want pictures again.
Will. I did this stuff before digital cameras and before I "joined" this club. No one except me ever gave a flying fig about the details onmy Pantera. Actually still no one wants to see pictures of the details in the family album. That would just make Uncle Doug weirder then he already is.