Ironhead Couple questions on over/under?

Hey, I just had a few questions for the experts. I have seen some ironhead bobbers that have that straight back bone right through to the hard tail. Am I right in assuming that is a certain strecth that gives it that look. I was told 4 inch stretch would usually give that look.
Also, if I replace the stock front end with a springer, what am I looking for to just have a replacement of stock. what over/under is that?
I have been looking for info on this subject but seem to have issue finding it.

I appreciate any help.