All of that is true re the sizing. It could well be that the knurling process wasn't effective. Although it's reckoned to hang onto the oil that does get down the guides it's usually done to tighten up the clearance and to save you from having new guides fitted. Better when bronze guides are used. If your valves are new and still have unworn stems I think that a new set of standard guides should cure the smoking. There has to be a small amount of oil going down the guides and the clearance is set to allow this along with the small changes due to heat. New guides means that the valves and seats will have to be recut, to line everything up again. If the valve seals have survived that long, after say 100k miles, they might help stop a bit of smoking but by then it'll be caused by some wear in the guides. B18 seals can be replaced easily at any time. B20 ones not so easy but you can hold the valves up with a length of nylon cord down the bore while you change them. The shorter B20 valve guide will allow more valve rock as it wears and benefits from a better seal. I believe there is a modern Ford seal that can be used there, better material. Someone here will know which it is.