I bought my Alcons from UUC motorwerks, they're 4 pot BBK's with calipers the same as used on the supercharged Ariel atoms. I'm using 328mmx28mm Performance Friction pin drive discs which are actually replacements for the E46 M3. This way it will accept 17 inch rims.
The PFC discs are superb, no warping whatsoever, even after running through several sets of track pads from Hawk.
Of course it will accept M3 discs as well, but given the stock diameter 325x25mm on your 3.0si they should work fine. It all depends on the pad thickness then. You can shim out the calipers with a shim less so it will fit the 325mm disc. I'm running the .680 pads, but if you use the .730 pads, they'll accept thinner discs as well. Contact Rob at UUC Motorwerks in the USA, to save on shipping, they can ship the BBK kits without discs
The PFC discs are superb, no warping whatsoever, even after running through several sets of track pads from Hawk.
Of course it will accept M3 discs as well, but given the stock diameter 325x25mm on your 3.0si they should work fine. It all depends on the pad thickness then. You can shim out the calipers with a shim less so it will fit the 325mm disc. I'm running the .680 pads, but if you use the .730 pads, they'll accept thinner discs as well. Contact Rob at UUC Motorwerks in the USA, to save on shipping, they can ship the BBK kits without discs