My advice would be to find one and take a template off it. A car museum would be a place to start. Contact a "T" owners club or association.
I believe it's the competitors responsibility for the dimensions. If protested it will be their, the owners, responsibility to prove their car is correct.
I would also take a picture and put it in your log book. If what you do to your radius fenders looks like a photo of a stock ones, I believe that would meet the intent of the rules as they are intended. Dan?
Good luck.....