I'd rather declare a bit early than a bit late. They're not going to ever get enough runs to be in a position to bowl us out and win. On a good batting pitch if we bat them out of the game they could dig in, take absolutely no risks and go for the draw. With uncertain weather on top of that, we could bat ourselves out of a win. Especially with a bowling attack that isn't likely to tear through them (if they batted with their heads in the 1st innings they'd have got 400).
It would be better to declare an hour before stumps, give them just enough hope that they don't shut up shop.