spill over vs overflow

spill over is not a noun while overflow is a noun.

Word	Noun	Adjective	Verb	Adverb
spill over	No	No	Yes	No
overflow	Yes	No	Yes	No

As verbs, spill over and overflow are synonyms defined as:

Other synonyms of spill over include bubble over.

spill over (noun)	overflow (noun)
	a large flow
	the occurrence of surplus liquid (as water) exceeding the limit or capacity

spill over (verb)	overflow (verb)
overflow with a certain feeling	overflow with a certain feeling
be disgorged	flow or run over (a limit or brim)