Wheel Spacers '99 to '02 and '03 to '04
I did a fair amount of research and wanted to make it easier for those with plans like mine. Or basically, I hope this can help...
I have a '99 Super Duty and I am switching my stock rims for 2011 stock rims.
2004 and earlier wheels have different backspacing than all '05 and up wheels, which is the main reason spacers are needed to run newer wheels on older rigs. Also '99 to '02 studs have a M14 x 2.0 coarse thread. '03 and up have M14 x 1.5 fine thread.
To do this(after lift/leveling kit) you need to space the front wheels out 2" so that the tires don't rub on the leaf springs.
It is a wide spread debate on whether spacers are good or not because on wear to the ball joints, bearings, and improper installation or bad wheel spacers. Super duty's have billet hubcentric wheels which means that the wheels center themselves around the hub, or on the spacer there will be a ring to hold the wheel centered on the spacer, as opposed to being lugcentric.
What I am using are 50MM Spacers, with the 8 on 170MM lug pattern, with M14 x 2.0 studs. Doing this you can use your original lug nuts as well. For 2003 to 2004 trucks you just need to get spacers with the M14 x 1.5 studs and it will work just fine. Read the instructions for inspecting the surfaces and torquing and you should be fine with running spacers, even if you are towing.
As far as buying them, I never came across a manufacturer that didn't have billet aluminum or steel parts with studs already pressed into them. There are a lot out there and the internet or local distributors can help you find what your looking for as well. Also, aluminum spacers will be fine for relatively small size tires, but with bigger lifts and serious meats I would feel better running billet steel spacers because of the strength of the steel.
Hope this helps! If I run into problems in the future I will post them.