How much to cut out for fenderwell headers?

I,ve purchased some fenderwell headers from Hedman part #(75110-66),these were custom order so there...on there way.Does anybody have pics or measurements for how much of the inner fender I have to cut-out?I,ve seen Blownfishes and 66340sedan,s(Keith)jobs and they look very clean.Any help with some measurements,pics would help greatly.Thanks

P.S These are going on my 1966 Dart with a 408 and P&P Eddy RPM,s.