Having full confidence in the security of my bar/stem combinations is high on my list of "must have".
I have Cinelli bars & stems that have been in service for decades w/o any hint of a problem, all 1A stem and #64x40 bars.
These were the go-to standard "back when" for road racing and I've never seen a failure in service, although I have seen crash damage.

Buying old hardware w/o knowing it's provenance and possible ham-handed install, lack of maintenance, crash damage or abuse is somewhat of a crap shoot, even if it's well known for reliability and long service life. Whatever happened to those bars took some doing, or not-doing, to get in that condition. I'd bin them and having determined that the stem specs out replace w/ another set of 26.4 Cinelli (after close inspection) and test ride to absolutely be sure they are safe.

As far as "shimming"?
Of course not, unless the shimming recommend-er wants to risk his/her teeth and clavicles on a vigorous series of tests involving high speed descents on rough roads, max braking, full-on sprinting, hitting pot holes at speed and standing in big gears on steep hills to get some skin, bone and dental work in the game.

-Bandera