I wouldn't go much past the rear window of the truck roof. You will need to account for hard turns with the 5th wheel...
This GOOGLE PHOTO shows about as far as I would install the KAYAKs. This will most likely require you to have a support frame coming up from the front truck bumper...
Photos from GOOGLE IMAGES
Roy Ken
My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
RETIRED DOAF/DON/DOD/CONTR RADIO TECH (42yrs)
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS