There really isn't much usable power above 5,000rpm on a stock B230F engine. There is a reason that your slushbox shifts when it does. I have an M46 in my 245 and I can tell you from experience that there is really no practical reason to revv the engine to 6000. Those last 1000 rpm's take longer to wind out than you would think, and you enter the next gear at a higher-than-ideal RPM. And as Gene mentions, it's probably not a good idea to run an old, high mileage engine up to the redline on a regular basis-unless you want to throw your #1 con-rod. But if you really want it to shift later, you can always adjust the kickdown cable.