Yes the hammock width is the same at the head and foot because it is based on the spreader bar which are both 36". the material that forms the arc beneath the spreader bars is narrower at the head so you have a flatter lay. It is wider at the feet because you need more space for your legs to move around; plus they don't feel the squeeze like your shoulders do and the lower your weight hangs beneath the more stable the hammock is. The spreader bar width to material forms the ratio which I refer to. So If you want to use a 39" width, you can calculate the material you need and altter your plans accordingly. The measurements I used are all based on trial and error from quite a few different people like Grizz, BER, etc.... and my own comfort during my multiple attempts at hammocks.
In the camo hammock (my first attempt in this style), I mis cut the material at the head end and sewed so gold material to extend it a bit. Making endcaps is a bit of a pain and it dawned on me that it would be easier to just extent the material to the full length of the triangles. A little weight gain, but a lot less effort to sew it up.