Good Luck! Get as much back spacing as possible. It sounds like you are going with cast. In that case... get as much offset as possible. You may have to switch over to 5x4.5 and get FWD wheels in order to tuck.
As far as how much to narrow the rear end it all depends on the wheels. The less backspacing you have the more you narrow the rear. If you narrow too much you will also have to rebuild you frame rails narrower. I do not have my Dakota here to see excatly how wide a wheel you can run before you have to narrow the frame rails.
22's in the rear of a b/d truck look dumb. You have to narrow the axle WAY too much. Theres a orange dakota on www.layinbody.com that is b/d on 22's. Looks like he too like 3 or 4 inches off EACH side of the axle. They have to be able to clear the top fo the bed when it rolls in.
thats exactly what i'm doing to my dak....i narrowed my frame rails already....next is the rear end. I got 20x8.5 KMC Tramps with a 45mm offset i redrilled my lug pattern to 5x4.5. Layin frame.....soon body. I'm goin with 20x8.5's in the front again and 22xthe widest i can get in the rear. i might think about sectioning the body/flare in the back and moving it up to the top of the bed rail so i don't have to narrow the rear end too much. might be different.....might be gay.....ITS PHOTO SHOP TIME!!!!!