Changing the third member will not have any affect on the lug pattern. The third member is only the part of the differential that holds the gears, and in the case of the Chevrolet trucks and cars of that era, is separately removable from the axle housing.
In order to change the bolt pattern, you will either have to have the existing axle shafts redrilled, or swap out the entire rear axle assembly for one of the proper width that has the desired bolt pattern.
