From what I understand the BS8 is a fairly rare car... You might need to look at tunes from previous versions of GT and extrapolate settings from them.
In GT4, the key about a stock Speed 8 was to drive it fast, and everything else would work itself out naturally. Though I've yet to get my grubby little hands on one in GT5, I'd imagine the same would still hold true.
Yer I got this monster last night and it is very taily even when close to the limit, I tried to stiffen up the rear a little like the other mid engine GT1's LMP1's group C's etc but no dice lol.