What do I have to do to put a 4 under spring on my xs?

[usp222]

Ive been searching pics and looking around to find what I want to do with my build, I ran across a couple of bobbers with 4 under springers on them and I just really like the look. How do I but one of these on my 650 without messing up the trail?

 

[jamesgs4]

a -4 springer is only 2 inches shorter than a stock xs front end, so if you lower the back of the bike as well, the trail shouldn't be that far off.