Rhyming conventions of Early Modern English

I think would and flood are or were rhyming pairs in some dialects of English. This is not surprising, as /ʊ/ (as in would) and /ʌ/ (as in flood) are similar vowel sounds. I think in some dialects of modern British English, the two vowels are merged. The general term for words that almost rhyme is called slant rhyme. Words that are spelled like they might rhyme but are not are called eye rhymes.

As for the second passage, I think it is just a rhyming pattern of AABC DDBC.

Yes. Words like FLOOD could still have a rounded vowel in some varieties of London English in the mid-to-late 17th century. There were varieties where the vowel in FLOOD shortened early in the 16th century and developed an unrounded vowel /ʌ/ by the middle of the 17th. But there were other speakers for whom matters were otherwise. The orthoepist Christopher Cooper (1687) is one of them. In describing his own preferred pronunciation, Cooper quite unambiguously has a vowel like /ʊ/ (the vowel of modern HOOD) in words like BLOOD and FLOOD.

If you want to hear me reading this poem aloud in a reconstruction of how Marvell's own English pronunciation may well have sounded, click here.