Is Bella good with treats & training? If so, have her sit somewhat near Diesel and then give treats all around. Then have her sit slightly closer to him, and then give treats! As long as she's not showing teeth or biting. Make sure she's distracted from him by something super yummy. Do this daily and slowly decrease the distance they are from each other. Don't force it though. Only have her sit as close as she can given that she gets a treat out of it.
If it's not working, give her something more yummy! It'll work eventually.
Scottie did
not like Casie when she first came home. I did this on a very regular basis until he would tolerate her existence and then tolerate her being within a meter from him, etc. Then, I only gave them treats if they'd sit still in the same dog bed. At first I was in awe that he didn't mind that they were side by side on the same dog bed. But they're pretty funny. There's a certain order it has to happen. He has to be on it first, and then he's okay if she joins the bed. If she's on the bed first, he somehow signals for her to get off, then he gets on, and then she gets on. LOL...
Anyhow, they still don't cuddle, but they both coexist now and will play with each other daily.
As others mentioned, it does take time. Each pup is different, so it could take Bella a lot longer. But, keep at it and make sure you positively reinforce her whenever she's near Diesel. Even if it's just for a brief moment.
If she does any kind of biting actions, make sure you tell her it's not okay. Do a firm "NO". You have to let her know that as the big human boss, you will not tolerate any violence between them. If you don't, it could potentially escalate into something more, which you don't want.
I'd also make sure to keep them separated when you're away. I've read enough sad stories about pets injuring each other while the humans are away. It's very scary... you wanna make sure to avoid this at all cost.