5 bolt conversion

[bigblock69]

I am restoring a 68 C-10 and want to convert the rear to 5 bolt. I have a 71 1/2 ton that has a five bolt drum to drum; however, it is 3 inches wider than the rear from the 68 which would make the axle tubes extend out 1.5 inches too far. The mounting purches are the same distance, which leads me to believe it will bolt in. Question is this. Why is it wider and can I use it? If I cannot use it, I will have to buy new axles and a carrier to go to 5 lug posi. Any information will be much appreciated.