What length of spokes are needed to lace....

an 07 Street Bob hub:



to a 40 spoke 16" rim:

 

I don't know the length but they will need to be custom cut by a reputable spoke maker. The front has two different size spoke heads that alternate to allow the spokes to pass over each other. I ordered mine from HDwheels and the guy knew exactly what I needed. I believe his name is Andre. Give him a call and he'll be able to help you out.

If you were to buy a set of spokes off the shelf they only come in one head size so they won't work. Found this out the hard way.

Good luck, it's a great mod.