Well, some progress..
Drivers side floor is now welded up, primed and sealed.
Next up!
Rear right chassis leg..
First cut the old off,
Good decision to cut the section out with the re-enforcement inside, it was all quite rusty inside.
Next, I cut a desk leg outer profile to fit, which is the perfect width(50mm) and thickness(2mm), weld in the anti squish bush which is a hollow spindle from a desk leg, and make the cutout for the three "fuel" lines.

I'm very pleased with the result of this.
Now the the other side, on the surface it doesn't look so bad.
But beneath two layers of steel I found this. This is an easy fix, so maybe I'll cut in the other side as well for a peace of mind.
I can't help but wonder, are there ANY totally rust free 5's around? I got this one fairly cheap, and knew there was rust to some extent, but not this bad obviously. I'm not too unhappy about it, because I know it's rustfree when done - and I've seen worse on the forums..