There's a few ways to adjust P90's
SGs of the 60's and SG Classics have dogear bottom plates, and use standard pickup screws and springs though the pickguard
You can put wood screws (JassBass screws work... sort of) into the body on thicker body guitars. SG's are not thick enough for this. The pickup will have springs on the screws and or foam under the bottom plate (like Fender Bass pickups).
There is a mounting plate seen on some of the Les Paul guitars, where that plate will accept either mini humbucker outer rings, or P90 screw spacing for adjustment. This is a really nice, but a little more expensive (and trickier to install) alternative.
Some Les Pauls have threaded bushings inset into the body, this is for p90 spacing only, not mini hum.
And fnially... what I did in my SG was to make up some wooden shims out of mahogany and screw those into the pickup cavity, then screw the P90 to those shims totally solid pickup-to-wood.
I use the springs, and that works great. I noticed that foam gives me an annoying vibration in the guitar.
