I recently did the same thing with my new to me 2001 265. The stock trays will not hold 27 batteries, but the trays I bought at Bass Pro fit the exact same holes, but did stick off the aft end of the battery shelf about 1-2"s, but even still the batteries (Interstate 27 Maint Free) are nice and secure strapped in place as before. As far as fitting in the top opening, it was slightly tighter but still easily done (granted you have to turn the battery almost 90degrees down, posts facing aft I think); but again a few try's and you're a pro at it, no worries at all.
Now that I think about, I think I'm gonna mount one if the extra 24's on the port side with an old tray and leave in there as an emergency spare, just charge it solo on occasion, guess it can't hurt, it's only 50#'s.
Goodluck, sure is an awesome boat, just need more time and opportunity to use it!