The 2008 tailgates will fit on a 1999 to 2007 as well.
Both a non-step & step will work and the procedure would be the same.
Hopefully, you could fine one that is already painted and matches your truck, or you have found one and all you will have to do is have it painted.
If you choose to piece it out and buy it at the parts counter, the tailgate assemblies are different between the two. There are several parts that you can use off of your stock tailgate and just transfer them over to the new one.
Your best bet is to find one and just have it painted to match in case it is a diff. color.
Look in my signature at my tailgate conversion and you can see what I did to make mine work.