I think there is no special info about this because its an easy task, you will need the tool called " wallmarket"
-open your KERNEL file(in FF7 directory) with wallmarket
-go in weapon section, if i remember well in the very bottom of the weapon section, there is the masamune
-u will see that only sephiroth can equip it, but u can set any other character for this weapon ,check the box "cloud"
-now go in initial data(still in wallmarket),u can choose any weapon that u will have from the beginning, just choose the masamune instead of buster sword for cloud
-patch ur kernel
Now when u ll start a new game cloud will have masamune equipped, this wont works if you load an old save game(before you patched it), you must start a new game
NOTE: this will works ONLY if you have a sephiroth patch,it wont works so easily if you have cloud's model ( in that case you need to edit the character himself)