I had the same thing happen to my 98 4.6, number seven plug fouling out. Code checked to confirm. changed plugs later the same plug fouled out. Turned out to be a head gasket.
With the overhead valve situation mechanix said it would cost around 700 dollars a side to fix.
Older truck was not going to happen. I put block seal in it ran it for 2 months with it in. took it out, replaced with coolant, have not had a problem since. And just before I did this it was getting really bad, smoke pouring out.
One thing I wish I would have done, when I did this was to by pass the heater core while block seal was in the engine,
The block seal for some reason clogged up my heater core and while the heater still worked it never was as good as it was before, did not get as warm. It did not affect the radiator.
Just a thought,