I've been researching wheel spacers due to coming across some really light and inexpensive race wheels made by Enkei. The offsets would require hubcentric spacers and the bores for the wheels are 72.6 mm.
In my search I found a company by the name of Motorsport Tech here in the USA that can custom make them and he uses Alcoa 6061t6 billet aluminum. Just thought I'd pass along the info.
In the mean time I came across some 2001 Corvette Z06 forged Speedline wheels for my car. They are only 1/2 lb heavier than the Enkei's and for $600 I couldn't pass.
I'm finally going to get 315's in the rear of my car