Identifying free space

I've been working periodically on my own edit of FF6 for several years now; the file itself has been extensively patched and edited in various ways, mostly via FF3usME and IPS though I've done a little bit of hex editing here and there despite my essentially negligible understanding thereof.  Right now, I'm trying to implement a change to the Row mechanic; I had a patch created with information I sourced here awhile ago, it works on a fresh FF3 ROM but the game crashes after a fight when I implement it onto my version. I assume this happens because the patch is replacing some essential piece of code in the game's data, though as yet I am unable to find a suitable empty space for the code.

The ROM is headerless and expanded for the purposes of lengthening the names of enemy special attacks in FF3usME, and perhaps in other ways. I want to know if there is a reliable means of identifying free space in the ROM as to avoid replacing essential code.  Any assistance in this regard will be greatly appreciated; forgive me if this information is provided elsewhere on the site, I was unable to locate an answer thus far.

Thank you!