Hmmmm...steam and antifreeze smell comin' up from the dash, ya say Matt?! Yep, my guess IS that your heater core has gone south on ya, and that your cooling system isn't sealed, causing your engine to run hot.
How about taking the hose that runs from the block and into the firewall and to the heater core and instead bypass it by just redirecting it immediately back into the block's heater inlet? I did this exact thing on my old '69 ElCamino's 350 when my heater core had given up the ghost and when I was havin' the exact same issues you're havin' with your F100 here. And after I did that, it ran much cooler.
(...in fact, I never did get around to installin' a new heater core on that baby before I sold it back in '06...SoCal never gets that cold, ya know)
