This statement by you is the most important concept in this thread.

If a person has doubts about the condition of the chain elongation then they are likely to have doubts about the condition of the more dangerous parts which rub against the chain... what if the prior owner rolled in a new chain 200,000 miles ago but did not replace the rails ?

Then no matter how well you measure and put in the proper offset key your engine is still in danger of calamity ...
and those instructions are not all that clear... lots of discussion in the archives...

All things considered on an old engine in a car you have not owned since it was new... put in a new chain and all the things which contact it ...