dont believe the hype.....you can daily drive with 40's!
Here is my opinion...and experience. If you want to run 40's go with a minimum 6" lift, I like the fabtech personally. There is a guy thats selling one for a damn good deal right now, check the classified section.
Yes, the 40's should work on your wheels....what the width of the wheels, 10 or 12". The 15.5's would look better ont he 12" wide Rockstars, but can definitely be run ont he 10" too.
Should you regear with 40's, probably. Do you need to regear for sure, not necessarily.
THings to consider....beef up the front: tie rods, pittman supports, etc.
Im running a Fabtech 6" lift with 20x12 Fuel wheels, Toyo Open Country 40x15.5x20's, im not regeared. Truck runs just fine so far around town and on the highway. I probably will regear eventually, but its not necessary right out of the gate...for me anyhow. I upgraded the fornt suspension items as well.
I had 38's on it before the 40's, to me it runs about the same as far as the gearing goes.
I say go for it, you will love it!