straight and narrow vs strait and narrow

straight and narrow and strait and narrow both are nouns.

Word	Noun	Adjective	Verb	Adverb
straight and narrow	Yes	No	No	No
strait and narrow	Yes	No	No	No

As nouns, straight and narrow and strait and narrow are synonyms defined as:

straight and narrow and strait and narrow: the way of proper and honest behavior

straight and narrow (noun)	strait and narrow (noun)
the way of proper and honest behavior	the way of proper and honest behavior