GTA sill repair

I found this picture from back in 2009! which is just like my problem - rust holes in inner sills

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What is the best way to do this without setting fire to the fibreglass, and hopefully avoid a respray.
Is it OK to just weld a patch over the hole rather than fully seam weld in an insert? Is it an MOT issue?
A few plug welds or full lap weld all round? Or just Araldite it on? It's not really structural, is it?
I can release the outer sill top and bottom (I think). Floor might be a bit more difficult. Would a bit of spacing be enough to protect the fibreglass?
Any helpful advice welcome.

Hi if you have got that problem you will probably find if you look underneath where the raised floor section is I think you will find the sill there will be rusty or probably have a hole in it or nearly.
The sills rust from the inside out, the GTA has a simple one peace sill sandwiched between the fiberglass and two side structures welded to it the seat section as in your picture and the jacking point at the back. It’s better to change the hole thing in one piece which you can do on the GTA.
You have to take out the seats and carpet part the fibreglass and adhesive bonds carefully with heat, there are two bolts at the front from memory and cut off back to sound metal the jack and seat members, and slowly push the sill down and the hole thing will slide out.
Then use that as a template to make a new one then slide it back in and re bond and weld the seat and jack sections together just like that.
Unless you have all the kit don’t attempt it plus the car needs to be supported properly.
Filling it with pug or welding in place is not a good idea, you could set light to your car if you weld there as it could burn in the inside of the sill that you could not put out.

Thanks for such a detailed reply. You didn't mention taking out the floor, which surely has to be done to weld the cross-members back together without setting fire? Or am I missing something?

That’s the only tricky bit you do have to weld the metal from under side but it’s not to bad small welds at a time with a compressed air gun near by and a fire extinguisher.
Remember when under tacking any thing like this you should have removed all Petrol from the car and disconnected the battery.
Again do not attempt this unless you have experience doing this sort of work and have the right equipment, this is NOT a DIY on your drive way job.