Aahhh... the classic big coupe!s8int said:
I spent dozens of hours on track in my E28. Since the E24 is all E28 mechanicals underneath, wheel and tire stuff should be similar.
I suggest a 245/40/17 on a set of 17x8.5 wheels on all 4 corners. Stay away from a 245/45/17 since the tire is about 3% taller. Offset is not as tricky in an E24 as an E28 since the 6 has more room due to slightly wider fenders. An E28 would need an offset of 20-25mm with possibly a 5 to 10mm spacer up front to clear the strut. You might be able to get away with a 15mm offset and no spacers in your 6.
Ed