For plain old spacers, I doubt the studs are long enough for much more than maybe 1/2".
What has already been mentioned about bearing load is more critical on semi-floating hubs; you can get away with a bit more on full-floating hubs, within reason.
The manufacturers, all three of them, have been using very wide spacers on all of the DRW 1-ton 4x4 trucks for many years; the front axle of a DRW 1-ton is no more than a 3/4-ton SRW axle/hub with this spacer added.
That being said, the only safe spacers are those that have a set of studs made onto the spacers and a shoulder for hub-centric wheels to ride on; you attach the spacer with your existing lug-nuts, then mount the wheels onto the spacers studs.