I have an 8 3/4 clutch type suregrip with 10 plates/washers, 5 per side. I see that some places have rebuild kits with 12 plates/washers. Why the extras and will they all fit? Anyone have info on the thicknesses and is there a way to measure the correct prelaod or crush needed?

I have rebuilt quite a few of these units and what steels and belville type washers depend on the thickness of the ones in your kit. You can use a combination of thick and thin to get the pressure you want. Doctordiff has a diagram I think saved in the archives on this site.
If you use all the thick ones in some kits it will be like a spool. I have used a combination of the new steels and one of the thin original ones.

Gus

The stock sure grip units usually have thin plate/steel/dished plate/steel/dished plate on each side of the unit. IIRC, reusing the thin plates only combined with a Dana 44/8.75 kit from Dr. Diff in the order above added .100 to the stock clutch/steel stack on each side using thin plate/steel/dished plate/steel/thick plate (double check me on the order). The main difference in the new kits compared to what was installed at Dana/Spicer is the lack of a splined belleville disc. The only belleville is a steel (which indexes to the carrier) which is good because the splined bellevilles are often cracked, sending metal fragments through the unit. Maybe one of the older guys knows more history on the topic and when it changed.

My GTS drives well and can launch pretty darn hard without issue. My reason reason for rebuilding it was a broken belleville spring (cupped disc) on the passenger side causing the car to drive left under power and dive right on deceleration in gear.