You should have a 3.91 in your 90 535iA. I am going with a 4.27 lsd in my 90 535iA and I don't think it will be anything close to "a real neck slapper" after the swap. I am hoping it won't be as much of a lard arse off the line that it is now. As for the revs at cruising, remember these cars were designed to cruise at 100 mph + so while it will be turning a little faster it won't be unacceptable. Good luck.