Making a standard WB Ford E 150

[MGL]

Later this summer I am planning on building a copy of a van I had in highschool, a standard wheelbase Ford E 150, based on the AMT Phantom van. According to Wikipedia (the definitive source for accurate information), The wheelbase is 138" and the overall length is 206.8" vs 124" wb and 186.8" length for the short wheelbase. This suggests that I need to add .3 inch to the chassis and between the wheels on the body, leaving .5" to be added after the rear wheels (at the body seem from the rear end cap?) Should I just seperate the roof from the start and add the extra length at the center where its flat and work the body seperately? Have any of you done this conversion before? Any advice would be welcome as this will be my most ambious project to date.