[E-Type] transmission question; starter question

My bet is that the “vagueness” is indeed due to worn shift lever bushings.
If this $15 repair is not carried out post-haste, you will definitely be
looking at a gearbox re-build in your very near future. Terrible way to
have to spend the holidays! If the trans is already out of the car,
re-build it anyway (even if just a “minor” re-do), but don’t neglect the
shift bushings. This problem is ever-present in 3.8 and V-12 cars as well,
and is cheap insurance to correct. Cheers!

PS. I agree that the starter problem should have nothing to do with the
“line bore”.
Look elsewhere, the shim plate being a good place to start. Also check that
the ferrules are in place between the shim and bell housing. I have seen
several bells that have had the starter bolt holes “wobbled” out in the
soft aluminum, giving a very imprecise location fit to the starter. The
bolt threads in the starter have been known to get buggered up as well.