Wow, great news ! I have always been a great fan of that track, modern and classic layout too, perhaps classic layout is even better, certainly has more soul in it. If you'd need any testing I could do it, I also slightly familiar with tracks modeling/modding for AC
Very nice of original author to give permission. I would say that the original author approval is the main thing, but most likely Slider916 has improved a lot of things if you also need his permission. I hope you will be able to contact him soon.
There was one terrible event some time ago. One guy, or maybe it was two guys, converted Targa Florio to AC, it looked pretty good from previews. The track had a lot of authors. First of all original authors for GPL, then authors of rF conversion, at the end one of the rF conversion guys has just said "NO". Well.... I hope such thing won't happen to you. I also hope that your conversion is proper