Lowered 86 C10 Question

[BrookelynnGreen]

I recently bought a 1986 SWB C10 pickup that was lowered by the previous owner. It is currently running 15" wheels with 295/50/R15 wheels all the way around. I am looking at the Torque Thrust II's and am considering 17"-20" wheels. The problem i'm encountering is What drop does my pickup have and what is the biggest wheels I can purchase that will fit good? I would definitely love to put the 20" TT2's on my Chevy but if I cant then ill settle for the 17's.

*****from the ground the the top on the fender is 27.75" front and back*****

THANKS IN ADVANCE!!!