When one is writing historical fiction of any kind, there comes a point in our research and decision making where it becomes time to deviate from history. The inspiration for this post came from a conversation with my roommate, who is currently writing a 1920's steampunk piece and was getting rather bogged down in inconsequential details.
At more than one point I looked at her and said, "You do know you're writing alt. history, right?"
I am an advocate of accuracy, good research and backing up your decisions with facts. That being said, when it comes to areas I am familiar with, I tend to write from that experience. Sometimes though, history does not cooperate with us. Either someone isn't where they need to be, something hasn't been invented yet--or you can't find any information to back up something you heard from your third-cousin six years ago.
In the roommate's case, it was difficulty confirming asthma medications in the 20's. She wasn't having any luck with specific ingredient lists, and was instead getting bogged down in the minutiae of it. She needed to move on from it, she needed to break from history. Especially given how much she had already changed historical events.
When you change historical timelines in anyway while creating your work, it's a bit ridiculous to assume that nothing else would change within that. So yes, perhaps that particular shade of yellow dye wasn't used until 1875 in our time, but whose to say some lab assistant is Sussex didn't spill something and accidentally create said dye four years earlier in your timeline?
When small details get in the way of the story, it's time to let go a little. I've been known to leave words out completely when I'm uncertain. Usually I'll plug in something like this (Insert X Here) so I can search for this missing area later during re-writes.
Don't be afraid to make changes. Don't be afraid to choose new paths. History, in this case, is your playground. Have fun with it.