Again, using the B110 reference above.this is just for numbers, not actual measurements made by anyone. 9" difference to get the total length needed.
Measure the distance between the upper mount (where the bump stop mounts) and the top of the leaf spring or plate if doing a U-bolt flip. Measured from the center of the eyelet hole or bolt and the bottom of the washer closest to the body of the shock.
IE (B110 ref) 17.5" measured, known 16.4", you'll have a 1.1" difference to add to the next measurement. If it doesn't reach fully collapsed, take your measurement and subtract the difference from the manufactures collapsed length. With everything installed and wheels off both sides jack up one side of the axle as far as you can to try to collapse the shock fully. The GM bump stop is apx 4" in length with about 3" of usable compression, see link below. How to measure the amount of spacer you need. There are also 5165 non reservoir and 7100 with reservoir options that come in many different lengths/travel and valving options if you want to go with Bilstein. Many other "2 inch" lift shocks are similar to the 5100/5160 length, FYI. Oddly enough the 5100's for 2" lifts are roughly the same or even slightly shorter, so your droop won't change and if lift it 2" you'll be that much closer to the extended travel at all times instead of the piston resting closer towards the center position. My setup with the B110's (16.34" to 27.28") I needed a little more of a spacer than a standard 5100/5160 (13.84" to 22.4"/14.22" to 24.06") as they are pretty much the same stock, if I recall the OEM TRD OR Bilsteins are 13.98" to 22.94" for 2nd gens. The washers/spacer length for the bump stop mounting is going to be based on the collapsed stroke of your shocks that you end up buying. Just incase you missed it in the research.
I'm not good at explaining things short and simple. Sorry, this reply got more wordy than I was expecting. Click to expand.Let us know what you think of the GM bump stops and how it goes for ya.