[bender]

I couldn't tell you exactly how much the anti-squat is as I haven't measured it myself - when I've adjusted mine I kept raising the rear block until it the suspension pin looked flat.

To do the mod you will have to modify the rear plate or the rear block - I don't believe there is enough adjustment by doing just the plate so I'd recommend both.

And if you don't mod the rear block, then when you put shims under it it will hit the motor plate and bend the bottom plate down as Woody has said.