OK, so I go out to the shop to look in a factory Ford shop manual I have for a '90 F-series truck. Look up specs for F-350 4x4 monobeam and it says: +0.3 deg positive (inward) to -0.2 deg neg (outward). I take that as +0.1 deg as the nominal "Ford" spec. This is with stock tires. So, I would think if the truck is on an alignment machine, these specs (in degrees) would stay the same with any tire size. But if you are using a tape measure, and measuring in inches, toe-in specs will be different with smaller/bigger tires.
I still say, you can't go wrong with 1/8". Good luck.
2005 F-350 CC, LB, 4x4, Dually, 6.0 Powerstroke, Torqueshift, 4:10 Gears. Gooseneck and rear pull hitches. "Truck Covers USA" roll up aluminum tonneau cover. Putco Boss Running Boards. Aluminum finned diff covers. Bilstein HD shocks. ProComp steering stabilizer. XRF ball joints. NC Hornet CCV mod. RCD egr delete kit. AirDog fuel system. Strictly Diesel's regulated fuel and coolant return lines kits. Amsoil oil bypass (by NUC Motorsports) and homebuilt coolant bypass kits. CFM intake elbow. ARP head studs with factory gaskets and all upgrades. FICM by Ed with Atlas 40 tune, DC Power 185 amp alternator, MBRP 304SS "Cool Dual" exhaust system. SCT Livewire tuner with pyrometer. Autometer 'A' pillar gauges (Boost/Trans temp/Pyrometer/Fuel Pressure). 850 CCA Motorcraft Batteries. Using Rotella T-6 synthetic oil. Upgraded degas tank and cap. Hayes deep aluminum trans pan. GoGo Diesel modified direct clutch solenoid. Retro Solutions HID head and fog lights. High idle mod. Jimmi-Jammers. +More.