Two things you should pay attention to. Here's a pic showing C1500 stock spindles vs. drop spindles. Notice the axle location - it's near the center of the casting on the drop spindle, giving you the 2" drop. On the stocker, it's down low closer to the lower ball joint mount. Shiny new one with the blue cap on the threads is the drop spindle.
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When looking at C1500 spindles (stock OR drop) there's a critical choice as well - regular or HD brakes. For the 1988-1990 regular cab trucks, many of them were "regular" brakes which take a 1" thick rotor and have a shorter axle. If the truck was optioned with "HD" brakes, or was an extended cab (got HD brakes by default) then the axle is a little longer and accommodates a 1.25" thick rotor. The reason I mentioned 1988-1990 is from 1991-up, GM used the HD brakes regardless - but the catch is, either spindles will bolt up to all years of the C1500 trucks whether appropriate for the brake package or not, so you have to watch out for which spindles you're getting.
I don't have the specific measurement for the axle off hand but someone else might.
Choosing 2/4 vs. 4/6 is easy for me, and most of us that are fans of dropped trucks. Go 4/6. 2/4 never feels/looks like enough. But again it comes down to what
you want from your truck.
Richard