Front Torque Box Replacement - tips, advice?

Front Torque Box Replacement

Greetings,
Long story short, PO patched the front torque boxes by poorly spot welding panels over the torque boxes. I removed the patches yesterday and was pretty shocked.
The left one's bottom plate is pretty much gone. I can see the replacement toe board up above so I'd say the top plate is gone too. The small piece of U shaped frame that goes through that area is missing the outer side and the other two are in bad shape. I can poke holes with a screwdriver in the rear panel (where the ebrake and fuel line go through).
Needless to say, the car isn't moving until I fix this.

The car is sitting on its wheels and I plan to use 4 jack stands to stabilize the left frame as I work on it (2 forward of the bad area, 2 rearward) first. I'll finish the left before working on the right side.

A couple questions -
1) Do I need to remove the fender to weld in the new torque box? Will it be a lot easier if I do?
2) Since the toeboards have already been replaced I'm wondering if I can weld in the new torque box properly without removing them?
3) Is it safe to assume that a 2 piece torque box is my only option (since the floor is in place, etc)?
4) Is 4 floor jacks, located as described, enough? I don't want this thing to move while I have the torque box and frame cut out...

THANKS!

2+2,
Thanks for the reply.

As for jack stand placement. I was thinking that since all the wheels are on blocks that I could just support the frame I am working on. One just forward and rearward of where I am cutting and then one at the front of the forward frame rail and one at the rear of the front floor support rail.
If I have a couple more, where would you suggest puting them for greatest effect?

Thanks.

Here's a picture of my left side torque box - after I took off the piece of thin sheetmetal the PO had tack welded over the rust.

There is no upper panel on the original torque box and as you can see, the bottom is pretty much gone too - as is some of the frame rail. I bought the torque boxes from NPD and a set of rails but I think I'll cut/fabricate/patch the rail and just use the NPD ones as paterns and then return them.

I got the largest chunk of rusted metal out yesterday but now need to go back and cut a few spot welds and grind away some beads to get it to where I can start fabricating.

Oh, and there's a nice 1/2" gap at the top between the top of the TB and the firewall. That will be addressed as well.

Here's what I cut out of the left side - The torque box, a chunk of the inner rocker, a chunk of the frame extension and a piece of the front frame where it fits inside the rear extension.

I got some .07" and .10" sheet stock that I'll cut, form and weld in. I'm going to weld in a 3/16" x 1.25" angle iron support inside the rail prior to welding on the 'skin' for a little extra support.