There has been much talk here in the US in the "aftermarket" circles about camshaft/lifter failures. These failures are associated with the reduction of ZDDP in engin oils.
If you don't heed the warning, you will have a failure. The only question is when. ZDDP adative is $10 a can. Which is cheaper a can of ZDDP or an entire new camshaft assembly?
There is an oil addative made to help with "gummed up" hydraulic lifters. The problem with it is basically an oil viscosity reduceder (kerosene). Ford recommended a 20-50 multiviscosity oil for the Boss 351. Do you want to reduce the viscosity of your oil to save a set of lifters?
Yes, you can loosen up the lifters by driving them. My experience with Ford V-8s is that is a waiste of time. The lifters are usually permanently collapsed.
If you can identify the lifter, yes you can replace just that one, BUT you have to pull the valve covers and intake manifold to do just one lifter, why not do them all?
The valve adjustment can't be simpler. You just bolt them down and you are done.
The camshaft lobes on the engines that I have seen with these conditions are usually very noticibly worn. Many are pitted.
I have changed just lifters, and valve train as described here, and found that there is still valvetrain noise because the camshaft lobes are worn excessively.
Incidentally. Little of this has to do with mileage. It has to do with damage from disuse and seems to be irreversable.
However. That is just my opinion from my personal experiences.