I wouldn't bother with sealing the floor... too much to do, and technically, you'll never get it sealed. Again, a bit too late.
Since your window framing is still a rough opening, there's a bit of a chance you could maybe add a 2x4 perimeter to each side of the wall, then tack a 2nd 2x4 to at least one side.
Then put something like 1x10's or 1x12's on the interior of the gape, securely screwing the 1x to the frame, and to the 2x4's.
Get your gap down to 3"-4" and you can seal it with a rubber/neoprene. I ended up using some left over shower pan liner and sealed it with SilenSeal;
