OP you must really hate your father to put him such an awkward position,
I can understand having to deal with a difficult step parent, it's really hard, but to exclude your father wife for such an occasion is going to cause upset in their household for a very long time,
"Hello darling, guess what?, my daughter doesn't want you to come to her wedding, you are not welcome at the major family event,
Yes it was the dog poo incident,you shouldn't of said anything, that and mother doesn't like you, and when you brought up the differences in christian christening, they all hated you, sorry but I'll have to go on my own, I suppose I have to, having screwed up the marriage between the girl's mother and me, I can't make a stand, they might start blanking me also."
When you have guests, that makes you a Host, a Host's job is to make guests feel as comfortable as possible,
by not inviting your father's wife, you are putting him in a very difficult position, making him very uncomfortable, not just on the day, but long into the future, if your father thinks you are intelligent, then he will know that you will be aware of the consequences of this action, and did it anyway,
Invite your step mother, make her feel comfortable, if none of your 80 guests are capable of making polite conversation, invite a few that are,
You have quite a few months before your wedding, why not arrange a weekend away with your step mother, doing something that interests her, you might get to know her, away from family pressure, your father obviously saw something in her.