I have a mate who is a welder at a local steel company. He got all the steel for me and did the welding
The steel I used for corner plates is 3mm thick and 95mm wide.
There are actually 2 different widths of corner plates if you look under the car.
60mm for the two plates which come off the square centre chassis to the front / back horizontals in the wheel arches (these are factory as I cut the horizontals in the wheel arches about half way between the square chassis and where the diagonal down tube meets it)
I created/opened up the holes on the 95mm corner plates so there is a hole in each inside corner big enough for me to stick my little finger through
I did this as the main problem seems to have been where crud has got over the corner plates and sat in the crevices.
Now I can poke my finger in and clean them out.
I used tipex to write the location on each corner plate then photographed it before cutting it.
I also measured everything very carefully using the square centre chassis as a point of reference.
When cutting, I used a 1mm cutting blade in the grinder and kept everything I took off so I could piece it together off the car in case I needed measurements.
Offcuts from the new plates were kept and used inside the car under the bolts as strengtheners as only farty little washers were used by the factory.
There is an awkward corner plate to the centre tube.
Its awkward because the car floor has a recess to fit over it. So measure this one carefully. You're see which one I mean because its a strange shape too.
Hope that helps.