You should try using FF4kster. It makes dialogue editing easy by changing the pointers automatically. I haven't toyed around much with Japanese versions of the game, but in the two US versions the dialogue banks are all in the same place in ROM, so FF4kster will work just fine regardless. It has the potential to break your battle system, though, if you're using a version it's not fully compatible with.
Backup your ROM and give it a shot.
If it screws up the battle system, the fix I've used is as follows:
1) Make two copies of the ROM. Rename them Copy1 and Copy2.
2) Open Copy1 in your hex editor. Jump to $01/8000 (or $01/8200 if the ROM has a header)
3) overwrite the entire section from $01/8000 to $01/FFFF (or $01/8200 - $02/01FF if the ROM has a header) with FFs. The entirety of the battle system, at least as much as FF4kster will edit, exists in these 0x8000 bytes
4) Create a patch, using (the now messed-up) Copy1 as the "original," and (the untouched) Copy2 as the "edited" version of the ROM.
You now have a patch that will fix your battle system! So now you can edit whatever you need to in FF4kster, and just apply this patch when you're done.
*Note that, in my experience, the only differences between the two US versions of the game exist in this block of code, but I have no experience hacking any Japanese version... So your experience may vary.