long since vs lang syne

long since is not a noun while lang syne is a noun.

Word	Noun	Adjective	Verb	Adverb
long since	No	No	No	Yes
lang syne	Yes	No	No	Yes

As adverbs, long since and lang syne are synonyms defined as:

Other synonyms of long since include langsyne, long ago.

long since (noun)	lang syne (noun)
	past times remembered with nostalgia

long since (adverb)	lang syne (adverb)
of the distant or comparatively distant past	of the distant or comparatively distant past