DanDon,
Don't really worry to much about the trailer length its more of what you can haul. I would seriously suggest you go with the F350 SRW 2WD instead of the F250. We have been pulling our 40ft'r for 8 years with a F350 SRW 2WD and it does a great job. Its interesting that the SRW 2WD, at least the 2008, has a greater load capacity than the 4WD. The 2WD F350 is 4,340 and the 4WD is 4,250. If you decide to go with the F250 it is a big jump down -- the 2WD is 3,240 and the 4WD is 2,950.
Good luck with your research.
bigdon68