Hi Adrian,
There should be no need to remove the wing stay brace, as I used bolts long enough to go through the new wingliner bracket, the wing stay brace, and then into the sidelight. I put a piece of thin soft-ish rubber packing between the wing brace and the wing sidelight mounting to prevent any metal to metal contact.
Just make sure that the wing isn't fouled by the brace, as if you have ali wings (like mine) they could easily be damaged.
(Also be careful that you don’t try and bend the wing stay brace, or you’ll have problems trying to line-up the bolt hole).
