When I went for the CT scan at 35 weeks, which I wish I hadn't done, but agrred with the Dr to make sure there was nothing crazily abnormal with my pelvis that wouldn't allow a baby to fit through, they could get a measurement of babys head as it was sitting in my pelvis. I figured that 2 weeks later it wouldnt have grown too much bigger and I had the confidence (with no professional support) to give natural a go. Had I gone to full term and didn't know weight of baby or size of his head, I probably wouldnt have had the confidence to defy the doctors... I know it's really hard to make the right decision. Sometimes I would just cry and cry because my heart was telling me that natural was the best and safest option, while all the professionals were telling me that I was risking myself and the baby, and because my husband wasnt hearing my heart and only hearing the doctors, he was really scared of me going natural too. It wasn't that "I" wanted a natural birth. I felt the baby NEEDED a natural birth - I was terrified something was going to go wrong during the c-section.
How about a trial labour? Let yourself go into labour, if you dont progress as quickly as you hoped etc, go for the c-section. While I was labouring, one of the OB's asked me to have an epidural sited so that WHEN i had to have my caesarean, I was all set up - he had a great deal of confidence in me... I didn't object to this at all though...
If you do give labour a go, a midwife really helped me feel that everything was going well. As I started pushing, sha made me get up onto my knees and she sat my hand on top of my belly, with each push I could feel him sliding downwards - which I definitely could not feel the first time around!
Good Luck. Just do what you feel is best for you and bub.