An alternative might be to get a complete donor truck from here. Rust free D2 trucks all over here cheap. I might be able to get one free or close to it. You could build it up then swap the body on your schedule. I'd be happy to go look at some trucks if you need eyes here.
I guess the purchase/shipping would be pricey. But you had have a ton of other spare parts too you could sell to help recouped the cost.

 

What was your plate thickness?
I understand the original is 2mm spot welded to internal angle supports in the four corners where the rust starts.

 

SO, wanted to say thank you to all that provided some good advice and guidance on how to proceed... I have gone and bit the bullet (so to speak) and had the repair carried out by a local shop (Rockware) it cost approximately 375.00 all said and done.

Here is what it looks like now...a bit better I believe. This is the link to the video -
Cheers!

 

I believe RN has both rear frame rails and rear bumper support all preweilded into one piece. Zach was showing it on here one day. I forget the $ but it wasnt bad . It had all the new mounting points,squared up just slide it in place weild it and go.

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Ya, I saw the rear section it was about 640.00 or so just for the part and then shipping / welding. So I figured I was ahead with this repair, although you might have a point with the new mounting points and square. Although I believe mine is square...

Just going to put a couple drain holes in the low points to be safe for the future and then probably prep the inside of the frame rails and soak with some sort of rust preventative - insurance for the future.

 

I do quite a bit of off-roading plus the Disco is one of the daily drivers for our family. I am planning to get rust-proofing protection for the vehicle. This is a Canadian franchise which a couple of hundred CAN$:

Pricing

This could be a preventative maintenance option and I believe there are similar franchises in the States which offer such services.