'Read More' link after 300 words, only on posts over 400 words?

i’ve been googling for a while and can’t find any code or plugin to do this (hope i’m just missing it). My site has lots of long-ish articles, and we like to post most of the body of the post in the homepage loop. however, if it gets really excessively long i’d like a “Read More…” type link placed automatically after X number of words. But i dont want there to ever be an instance where after the ‘read more’ link there’s only like one or two more sentences, making the ‘read more’ irrelevant and kind of annoying for readers.

so i want something that will count the words in the post and if they exceed 400, will place the ‘read more’ at the 300th word instead.

possible?