While I wait to get my stuff to together to starting working on the frame of my car, my mind has been drifting to other parts of the car...most complex being the floor pan and the firewall.

Here is what i have and my skill level:
I do have a 220v MIG welder(Lincoln) and TIG welder (Miller MaxStar150 and small argon bottle), but only have real basic knowledge of welding.

Firewall wise, I'm really interested in one of these...
http://realdealsteel.com/i-20900998-...-firewall.html
http://realdealsteel.com/i-20145067-...-firewall.html

I guess my question here is doing a replacement of the whole firewall vs trying to fill in the holes is a better way to go for a beginner fabricator. This started to come up in my mind when thinking about how to mount the steering column & replace the master cylinder. It seems many need to fill in the steering column hole in the firewall from the tear drop shape to a circle for the mounting. My car also had a treadlevac so I have a Star Trek symbol like outline of bolts on the firewall from the backing plate (not to mention the firewall was rattle bombed black years ago and peeling). The intention is to later install raingear and have hidden A/C / heater lines so there are more parts to fill in or have holes.

I really like the recessed smoothie firewall, but wonder how easy it would be to screw up as well...I really want to do this myself as well but wonder if this is a job is too advanced to try to tackle. Being the fact that this would be so visible is another concern...

The other firewall they make sounds "easier" as they mentions installs at factory spot welds.

The floor pan, from what I remember is primarily the bad area under the rear seat.
This is the part I'm looking at...
http://realdealsteel.com/i-20145093-...th-braces.html

I guess I'm thinking how to go about this. Mounted to the frame? then cut out the old floor, brace the door jambs then weld the new floor in? Writing it out like that makes it sound easy....Being a beginner I really have no clue on the details of putting something like this in. Can someone fill me in?

Exhaust -
I will be working with 304 stainless. I was planning to fusion weld it only where I needed to. Ie flanges for the header, hangers and some of the bends needed to go up and over the differential. Would fusion welding be sufficient? The piping I have is 16 gauge. Should I use filler rod?

What's everyone's thoughts/suggestions?