You and 25,000 of your closest friends.
The best parade-waching spot is from the rail at the upper level of the train station. But I'd say our chances of getting a spot up there (and doing other things in MK that day) are pretty slim.
Otherwise, any spot along the street will be about as good as any other.
As for how early, that depends on how soon the other 25,000 guests start getting ready.
Could be an hour, could be three hours.
Heck, I've seen folks stake out a spot 4-5 hours before the regular Spectromagic Parade.
There are a lot of folks who want to view at that area of the park.