For everyone searching for quite the exact Slash sound of Guns N' Roses - Sweet Child O' Mine. Here it is. (I've been on this for a very long time):

Gear: I personally use one of his signature guitars (the Rosso Corsa) but it's fine if you grab a Les Paul ( Standard preferred ).
Rreally important: if you want to nail it you will need the Seymour Duncan APH-2S Alnico II Pickups he uses. (He has implemented them in almost all of his
Amp: Take an SL2555 or a silver jubilee (JCM 25/50 2555(X)) always in Lead mode (do not use the Powers Soak to 50 Watts it will ruin your tone)
Effects: Wha Wha: take a Cry Baby. (I took the SC-95, it has a larger range and a little boost) in front of the Amp
ZVEX Box of Rock (this little Baby imitates a JTM 45 pretty perfect and is used to kick the amp's ass giving you enough gain)
(just additional -> no must have: u could use a very slight delay if you like it for the solo and a noise gate (I can recommend MXR 135 Smart Gate))

Wirering: Guitar -> WhaWha -> Box of Rock (Rock and Boost Engine activated) -> Amp -> Box (I use an old 1960 Vintage (AV) Box with Vintage 30s)

The Settings: Marshall 2555: Presence: 8
Bass: 10
Middle: 7,5
Treble: 6,5
Output Master: The louder the better but be careful u have 100 Watts there
Lead Master: 10 (this is really important and takes direct influence on the sound. So please do not try to turn Output Master
up and Lead Master down (Lead Master must be 10))
Input Gain: 10

ZVEX Box of Rock: Drive: about 4 O'CLOCK!!!
Tone: 1 o'clock
Volume: 12 o'clock (very little less)
Boost: 12 to 13 o'clock

Guitar: Use the neck Pickup for the Intro and first 2 Solos: Vol: 10
Treble: 0

The main Solo: first Neck Pickup : Vol: 10
Treble: 3 (so you can work with the wha wha from 3:51)

from 4:04 switch to the Bridge: Vol: 10
Treble: 10

I hope I could help you. If you have questions I would be happy to answer them.