Did your temperature gauge indicate that the engine was running hot? Could have a stuck thermostat, or given your comment about stop leak and a cold radiator cap - perhaps a blocked radiator core. White smoke can be a sign that the engine is running way hot - but don't understand why you see it so soon after starting up.
By upper intake gasket - I'm assuming you mean the gasket between the curved intake plenum and the 'conventional' intake manifold? If so - not seeing how the work you did would cause what you're seeing, unless there's something going on with the coolant going to the throttle body - a leak that is somehow getting sucked into the manifold area that would be common to both sides.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 5K miles.