Outer modern derailleur chain ring wont work well flipped over,
if you buy a non ramped , single speed chainring, it will work
albeit up shift is a bit less automatic.

A friction shifted derailleur will have your hand accommodating the shift feel change,

big ka-thunk of STI shifting may not go as well ..

as to Thickness of spacers, get out your measuring tools,
you can determine much thru careful measurement ..