I think a conehead can just be a sign of prolonged ruptured membranes with an engaged baby, not anything to do with your pelvic arch. My first was more than an inch longer the first time she was measured compared to her length two days later. She had a massive conehead. My water broke before I went into labor, she was engaged the whole time, and all of that pressure on her head just made her head mold like crazy. My other two didn't have that because my water was broken much much closer to when they were born.
IMO, your biggest risk factor for having a repeat cesarean (and just about every cesarean performed in this country) has nothing to do with your anatomy. Your doctor and his POV regarding your chances is probably the biggest determining factor in the outcome of your labor. It's impossible for him to really know how well your baby will fit through your pelvis, but if he believes you can't birth a baby vaginally his actions during your labor will reflect that.
My advice? Stay active and upright in labor and especially during pushing. Avoid an epidural so that you can get into optimal vertical pushing positions. The shape and the amount of room in your pelvis is positionally determined, and there's a lot less room for baby to fit through when you're on your back compared to when you are upright (and if you're numb, you don't have the same options WRT positioning that you do if you've got full control over your body.
In addition to your position, your baby's positioning also can play a huge role in how a baby fits through your pelvis. The baby's head circumference is actually bigger if his chin is up or if he is posterior. Spending some time in positions that promote optimal fetal positioning (all fours, etc.) right now is a good use of your time and is usually pretty comfortable and soothing for your back at this stage of pregnancy.
Take your time getting to the hospital, trust your body, talk to your baby, and relax. He'll be here so very soon!
Here's a hopeful quote from this link http://www.ican-online.org/vbac/ceph...proportion-cpd
"Studies report two-thirds of women will have a successful VBAC despite a previous diagnosis of CPD. One study showed an 80% VBAC success rate for women who had undergone a cesarean for arrest during the second stage of labor (CPD). In another study of women who had undergone two previous cesareans for CPD/FTP, 56% delivered vaginally"