DO you mean William 15 September / 26 December start?
If he got 'The Conqueror' nickname, I assume you probably in the 26 Dec start so...
1st of all you got tons of money, use it to create Duchies title and put loyal Norman Dukes in your council.
Notable Dukes that can be created quickly is :
1. Give away Marsan to your 1st son Robert, along with the d_of Normandy.
2. D_of Kent and give it to your half brother.
3. D_of West Anglia and give it to Norman Count.
4. Hwicce to 1 of the Count that hold 2 counties.
Once that done, put them all in the Council (except Robert of course since he is still minor). Now your Anglo-Saxon duke that left is Duke Leicester and Duke of York, they will resent you especially Duke of Leicester since 90& of his de-jure count is under you. DO NOT transfer vassal to Duke of Leicester or giving them any county, but instead start revoking their county which can be done easily since you're surrounded by loyal Norman Dukes. If their rebel then good, you can strip another title afterwards since they are traitor. After that "little housekeeping" playing as William should be easy peasy. I tested it myself, and no trouble so far (40 years in-game, they stay quiet even when succession came, when the Lame Robert succeeded William).
About favor, 1 trick that can be done is buy favour cheaply to your target Count, give him duchy title, then put him in your Council. He will follow you obediently. Although I rarely buy favour since they are loyal to me, and 1 Duke that somewhat inclined to reject, owed me a favor since I resolved his problem with his wife