I've just set up an old Squier P bass with the 4 heaviest from a 4 string set and tuned it to drop A#.
I had to drill out the bridge for the heavier B string and file the nut. The truss rod needed a tweak as well.
The other problem I had was the low string sounded muddier than a very muddy thing. I replaced the pickups with some cheap Tonerider pickups and now it sounds pretty decent.