The 2WD have 3 leaves plus and overload leaf. The 4WD have 4 leaves plus and overload leaf. The 2WD are the
most desirable because they are softer with only 3 main leaves and achieve better flex...
Allthough you could just pull a leaf out of the 4WD and it would be similar....
You should get about 2-3" of lift out of the chevys, but the springs are very soft, so most people end up using lift blocks.
So you do not need to run a 9 inch shackle, they should match up close to the 3inchers that you have, but since they don't just run some 2inch blocks or so... That is real common... The springs are thick so you don't need to worry much about axle wrap...
If you do end up with axle wrap which is unlikey, then you can just leave the overload on, or cut the rear of the overload off and leave about 7-8" of it in the front of the axle and that will really help and you will gain a little more flex then compared to having the whole overload on....
Or just through a Traction bar on and call it a day...
But like I said you should have no problem, with axle wrap, just throw some blocks under and go wheelin...
