I did many a pork butt on my Weber 22.5" OTG before I got my first WSM. First, I suggest you do this:
http://www.hotsmokebbq.com/002gear/weber_bottom_vent_marking.php
Then, I bought a pair of thin firebricks. I stand them on edge on top of my charcoal grate and separate the area in two. I dump one chimney (unlit) behind the bricks and put a throw-away aluminum pan on the other side of the grate resting on the charcoal crate underneath my meat. I then put twenty lit briquettes evenly on top of the unlit. I put about three apple wood chunks on top of the fire, for smoke. Put the food grate on and adjust the bottom vents to about half open (monitor the temperature). You can cook at about 275-300 degrees. Put the meat on when you are up to temp.
I apply my rub and let it set before starting my fire. I inject my Boston Butts (check out Harry Soo's pdf on this site for a injection formula) before cooking. When the internal temperature hits 165 degrees, I wrap the butt with aluminum foil (I apply Harry Soo's mixture as per the pdf before sealing it up). Then, back on the fire until it is probe tender. I gently open the aluminum after the butt hit's 195 degrees internally. Sometime after 195 degrees up to as much as 205 degrees the meat will be tender. I use a fork, insert, and try to twist it. When you can easily insert and twist, then pull the meat and rest it a bit after opening the aluminum (you want to stop cooking - if you leave it sealed up it will continue to cook from internal heat) before pulling it.
You can do a FINE job on the Weber grill, it just takes more attention and fire tending than the WSM.
Keep on smokin',
Dale53