Shocks are measured my collapsed and extended length. Anything other than that is meaningless.
So you know it is lifted? Most aftermarket shocks are good for 0-2.5" or 3-6". You should be able to tell if it is more or less than 3".
Thats probably not much help though.
If you are basically measuring the rear block under the spring, your 3.5" sounds like someone swapped in the F350 SRW blocks. Not really a lift in the big scheme of things. Use shocks that are for OEM or no more than 3" of lift.
Just to add some more info (double check your ride before using as guidance):
Stock – ride height = 19” (eye to upper bracket), Max compressed length = 15.5”
Stock - ride height = 24”, max compressed length bottomed out on bump stops = 20.2”
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