At Discount Ramps.Com we have sold thousands of ATV ramps. We have found the following bed heights and ramp lengths work the best:
use Ramp Length
32 inches use 71 inches
35-36 inches use 82 inches
36+ use 95 inches
If you go the 8 foot length you will probably need to use dual runners. For the shorter lengths you should have bi-fold and tri-fold choices.
Your F350 sits about 36 tall so on level ground you would want an 8 foot ramp. You can get by with something shorter if you can load off a curb or put your rear tires in a dip.
Let me know if I can help with anything else.