My brother in-law has a 1956 f100 he is changing spark plugs and has one that turns about a 1/4 then locks up he has put quit a bit of tork to get it out but dosen't want to( over do it )anyone had this happen to them Thanks, Michael

 

I'd spray the crap out of it with penetrating oil and let it sit for a bit. If that doesn't work... heat.

 

Could be some carbon on threads, most common if threads are a bit longer than stock. Have him try screwing it back in, start engine and drive about 10 miles or so. Then try to remove it immediatly after shutting the engine down.