You CAN switch between A and B Spec in the pits (always good to drive for an hour or two to get a big lead, then let B-Spec Bella take over).
You CANNOT tell Bella to pit in at a pre-determined time (ie 6 laps). You will have to tell her every time - it's to stop you from letting it run overnight to get money. Plus, the game sometimes switches back to A-Spec for some reason (or so I've heard). It should take you about 8 hours to run in B-Spec at 3X speed, so if you have a day off, just let it run and do something else while you keep an eye on it. At least, that's what I plan to do when I get out of hospital and won't be able to move much for a few days.