I've had this Mexican strat for a long time now and it looks and feels great but I have been disappointed in the sound and therefore I don't play it as much. It has 3 single coil pickups and I was hoping to put a SD P-rail in but I don't want to invest the money if they won't fit. I looked under the pickguard and it looks like a p-90 would fit in the bridge position. So I guess my question is if the pickup would fit in the bridge space without modification and if it would fit in the standard HSS pickguard?
Yes. If the body is routed for a Humbucker in the bridge, all you would have to do is get a HSS pickguard and slap a humbucker in, or you could just get s stacked single coil and use that without having to get a new pickguard. The stacked singles don't sound that much different to a full size humbucker IMHO.