OK, there's a lot blank. Go with the wakizasha for the weapon attack?
I'm currently favouring the Medium katana for 1d10 base damage as that's what the original monster preferred. It wouldn't be hard for a supermonster in D&D Fantasy Japan to find enough human-scale katanas for all its limbs.
It's important to nail down what weapons its tentacles can wield and express that as a Special Attack ability or in the Tactics flavour text.
My initial idea was they can wield up to a Medium 2H weapon (e.g. human-scale greatsword), a Large standard weapon (e.g. ogre-scale longsword) or a Huge light weapon (e.g. giant-scale shortsword) so the base damage maxes out at around 2d6, or 2d8 if it's lucky enough to acquire a Large katana from an Oni or the like.
What do you think about the tentacle reach? Gargantuan kraken have 60 ft tentacles. Want to boost that to 80 or 100?
Well all the illustrations show the face-tentacles as being too short to reach the ground so unless they can stretch out like elastic they can't be 100 feet long.
OA7 Test of the Samurai has two illustrations, both of which show tentacles roughly as long as its humanoid arms. Which'd suggest a reach of 30 or 40 feet if the height is 100 feet. The flavour text in encounter
27. Guardian’s Cavern mentions "each of its seven
30 foot long tentacles" which supports the former.
MC6 - Monstrous Compendium Kara-Tur Appendix shows tentacles about the same length, or 30-40 feet, but the flavour text has "Seven tentacles extend from its head, each
nearly 20 feet long."
Come to think of it, if its 80 to 100 feet tall could its regular Space and Reach be higher than the usual 30 ft./30 ft.? A hundred feet is a good 50% bigger than the base 64 ft. of a Colossal creature and they're supposed to be one of the biggest monsters in Kara-Tur with only the biggest Gargantua exceeding their size.
We could make their Space/Reach 40/40 since a Humanoid Gargantua is 50/50 and a Reptilian Gargantua 50/40.
Maybe we could have the Females be 40 ft. space and around 100 feet tall and the Males 30 ft. Space and 80 feet tall. Female krakentua are a lot more powerful than males so they might be significantly bigger.