I would say that fully 80% of the Cougars that are no longer on the road, died because of metal mouse.
You have options. In increasing order of preference (for me):
1) If you have room/time, part it out completely and scrap the shell. You won't make your money back, but you may well save some other cars with components that are straight-swap or can be refurbished.
2) Take her to a welder and get a quote. Figure at least Ā£150 per corner for fabrication and welding of sill parts. Arches might be more because of the double compound curves.
3) Talk to Topcat (Tony). He produces pre-formed sill panels which, with a little trimming, can be welded straight in. It's possible he's started doing arch kits as well, but you'd have to ask him. Buying the ones you need (and finding a friendly welder to fit them) may well save you a few hundred quid, depending on how bad the damage is.
But before you do any of that, you need to get under there and assess just how bad it is. You already know that she's not going through the next MOT, so find out just how extensive the rot is, paying special attention to the metal just behind the front wheels and in front of the rear ones. Take a hammer - claw, ball-peen, or tapping - and bang around until you find the edges of it. Don't decide anything until you've done that.
