One of the great things about a J-bass IMHO is that there are SO many pickup options. (I tried 4 different sets in my last one...)
I suspect to make a SCPB fit, you'll have to do some routing (unless it's one of those J bodies with the larger rectangular routs for the neck pickup).
Rather than do that, if you're going to be buying a pickup anyway, I wonder if Carey might be willing to make a custom quasi-SCPB in a J housing? If so, he could wind it to be compatible with the bridge pickup you already have and the two could hum-cancel each other.
Just a thought... have fun tinkering! (I always do!)
