You really need some vertical separation to get away from that much power. As I'm sure you know, running 250w in close proximity to the scanner antenna will likely result in too much energy hitting the front end of the scanner, possibly damaging it.
The only way to really know for sure is to set up the scanner antenna, hook it up to a power meter and a 50Ω dummy load and try transmitting with the CB/amp. That'll at least show you how much power is going to make it down to the scanner.
How much your scanner will handle would be unknown.
If your scanning doesn't require listening to frequencies below about 50MHz, you could probably set up a filter to block out some of the signal.