True, quads were valved differently, and I considered adding them to our K20 (single shock model). Decided it would only be for looks and dropped it.
I did spend waaay more time finding the right shocks and steering stabilizer that fit without any modifications to the truck or shocks, then it should have taken.
My takeaway from that in synopsis. DON'T use Bilsteins, not for the lack of quality, but their "applications" that fit, do not fit. Different reasons for each type, F, R, stabilizer. You can make them fit but not worth the hassle, IMO.
Skyjacker does make shocks and SS that do fit perfectly out of the box and offer lengths at least up to what would fit 6-8" lift based on my research. But measure everything.
Here's what Skyjacker parts fit mine for part numbers (non quad front of course) 4" lift K20 C6P rear suspenion
Steering stabilizer (OE single stabilizer) - 7055
Front - 8071 15.57" - 26.07" These were one size too long. They'll work fine, but I'd go next size shorter. I have just over 20" eye to eye sitting at rest and without measuring max droop on a RTI ramp etc, the next shorter size appeared a tad short. Almost flip a coin difference.
Rear - 7083 17.32"-29.83" I have 26" eye to eye sitting at rest (Which equates more to a 6" K20 stock spring block lift or a 4" K30 stock spring, likely due to the C6P heavy payload package.
Thats why I recommend measuring and looking them up. Skyjacker product data charts are very complete so easy to figure what you need.
Hope this helps