Python searching a large list speed

I have run into a speed issue searching through a very large list. I have a file with a lot of errors and very strange words in it. I am trying to use difflib to find the closest match in a dictionary file I have that has 650,000 words in it. This approach below works really well but is very very slow and I was wondering if there is a better way to approach this problem. This is the code:

from difflib import SequenceMatcher
headWordList = [ #This is a list of 650,000 words]


openFile = open("sentences.txt","r")

for line in openFile:
    sentenceList.append[line]

percentage = 0
count = 0

for y in sentenceList:
      if y not in headwordList:

         for x in headwordList:
             m = SequenceMatcher(None, y.lower(), x)

             if m.ratio() > percentage:
                 percentage = m.ratio()

                 word = x

         if percentage > 0.86:        
             sentenceList[count] = word
count=count+1

Thanks for the help, software engineering is not even close to my strong suit. Much appreciated.