Got to know backspace. If you have the wheels and tires, get them in the wells, leveled/centered and measure required overall length. Best way to do it and no mistakes.
Crackedback is right. It is the best way, because of back spacing. Measure, measure and measure more. What ever you take out of the housing will also be taken out of the axles. Are you having your axles resplined or ordering custom axles. I would recommend ordering axles, because it is only like $100 more and you get much stronger axles than stock. If you aren't removing more than 3"-4" they won't be able to respline them because of the step that is on the splined end of the shaft. So I have been told.
