I know this thread is old, but I've been looking for the same answer as the OP in order to try to improve my own "record" of 1606 AD. I didn't expect to be the fastest of those who answered though, since I hoped to beat my record once again (the 1606 AD game improved my previous attempt that got me to Alpha Centauri around 1700 AD).
If it matters, I played as Qin Shi Huang of the Chinese (Noble/Marathon) in BTS 3.19 with K-Mod 1.44b (a mod that improves the AI, among other things) on a slightly customized (start position) 'Gigantic Accurate Earth Map (232x112) for Civilization IV' version 1.0 created by Laskaris. I also played with a few self-imposed limitations:
- no slavery/whip for my free citizens, no pacifism either (I see them as game exploits for the human player, just as higher difficulties are bonus driven exploits for the AI)
- no offensive war, except 2-3 small bursts of blitzkrieg to secure 1-2 previously voluntary vassals for Mining Inc. in the late game by capitulation
- all cities conquered in the above blitzkrieg bursts were 'liberated' to the original civilization once the vassal was secured after a couple of turns
- all the traded techs and corporation resources were always secured through fair negotiation and not by intimidation or threats
- no techs were acquired by forcefully instruct vassals to research them
I did, however, have a great start (Gold, Silver and plenty of food in my capital BFC) - as I should have had as the Chinese - and obviously a good knowledge of the map, bar where the resources were located (since it was the actual Earth's map and geography is one of my hobbies).
tiles and 6-7 
tiles just laying around everywhere on the map, able to be improved as soon as they're claimed. Such a ridiculous amount of food and production will easily catapult you into space-win dates that are normally only achievable on marathon speed with HC or Darius. In my save by the end of the game I think I had 40 gold resources and 40 wheat improved...absolutely ludicrous.
