Side notes about the 120 years:
In the Book of Jubilees V,6ff, the sentence seems to be applied to the giants rather than ordinary men, and disconnected from the Flood:
And against the angels whom He had sent upon the earth, He was exceedingly wroth, and He gave commandment to root them out of all their dominion, and He bade us to bind them in the depths of the earth, and behold they are bound in the midst of them, and are (kept) separate. And against their sons went forth a command from before His face that they should be smitten with the sword, and be removed from under heaven. And He said 'My spirit shall not always abide on man; for they also are flesh and their days shall be one hundred and twenty years'. And He sent His sword into their midst that each should slay his neighbour, and they began to slay each other till they all fell by the sword and were destroyed from the earth. And their fathers were witnesses (of their destruction), and after this they were bound in the depths of the earth for ever, until the day of the great condemnation, when judgment is executed on all those who have corrupted their ways and their works before the Lord. And He destroyed all from their places, and there was not left one of them whom He judged not according to all their wickedness.
In the Book of Biblical Antiquities IX,8, the connection with Moses' lifespan is explicit:
I the Lord will kindle for him my lamp to dwell in him, and will show him my covenant which no man hath seen, and manifest to him my great excellency, and my justice and judgments and will shine for him a perpetual light. For in ancient days I thought of him, saying: My spirit shall not be a mediator among these men for ever, for they are flesh, and their days shall be 120 years.