One of the hallmarks of traditional steampunk is that it's a re-imagining of the mid to late 1800s or early 1900s, when steam power was the dominant technology. Tied in with this alteration in technology is changes to the historical record. The Difference Engine, for example, has Lord George Gordon Byron (who didn't die in the Greek War of Independence in that world) as prime minister in the 1850s, and a United States of America that's fragmented into several constituent parts -- the Republic of Texas, the Republic of California, the United States of America, and the Confederate States of America are the largest.
Whenever you alter history, you run into the problem of making historical characters act the way they would normally act. Harry Turtledove, one of the alternate history masters, has several series where he adds "watershed" events that alter the course of history. In the Worldwar and Colonization novels, he posits what would happen if aliens invaded Earth in the middle of World War II. In what's called the Southern Victory or Timeline-191 series (not officially, mind you), he considers what would happen if the Confederacy was victorious in the American Civil War.
In How Few Remain, Turtledove includes Samuel Clemens as a sharp-witted newspaper editor in San Francisco of the 1880s. Clemens, best known as Mark Twain, did in fact work in San Francisco in the 1800s -- except he was there in 1864, and was living in Hartford, Conn., during the time frame Turtledove's story occurred. The problem with doing this, of course, is that Turtledove has to have his fictional Clemens react to events in the same, or similar way, the real Clemens would have.
The above is why I've tended to shy away from historical fiction/alternate history, besides my previously stated fear that I'd get drawn too far into research. The possibility that someone would read my story and say "so and so would never do that" for a historical character has previously been too great for my liking. Of course, that also changes as the documentation on a particular person increases. For the major figures of history -- Abraham Lincoln, Charlemagne, Napoleon Bonaparte, etc. -- you can find enough extant scholarship that getting them right is only a matter of reading enough.
I have tremendous respect for the folks who build careers from alternate histories or straight historicals. My hat's off to their research prowess, which far outstrips my own. Those people are the real research gurus.