You don't say if you have a 4x4, 4x2, cab config, Lariat ?, interior, PW, PDL, Auto/Manual, etc...... plus the value will vary greatly across the nation. Go to New Car Prices | Used Car Values - Official Kelley Blue Book Site
and look it up. In my area I haven't seen a clean 7.3 sell for any where near KBB pricing, they easily go for several thousands above their quote, so make sure you check local prices in your own area. Your mods aren't going to gain you much as most folks won't know the value. The 7.3 are actually staying very strong right now, the 6.0 has a horrible reputation and nobody wants to gamble on the 6.4 so many are looking for the 7.3. Yours hasn't reached 50% of the expected life of a PSD, I wouldn't sell mine for double KBB and I am 100% serious about that!!!