Pre-clutch tube repair questions, Need some advice from welders Options

[ThinAir]

I'm getting ready to fix the clutch tube on my '73. It's not just loose like your typical clutch tube repair. In my case the firewall is broken away in about a 2x3" rectangle and I can see that the rear 3-4" of tube are gone along with the firewall section.

I've got a parts car that I need to start cutting up. My plan is to cut the center tunnel along each side where it meets the floor panels so that I can remove the whole tunnel. Once I get it out I'll drill out the spot welds so that I can get to the tube from the open bottom. Once I get that far I should be able to separate the tube from the side of the tunnel so that I can move it to the '73.

So I've got 2 questions:
1) Is this a good plan?
1a) Is there a source for a "new" clutch tube that I should use instead of trying to get this one out of the parts car?

2) What kind of spot weld drill works best? Here's one type that I've seen: