PersonalizeWord_1.0.sit should help. I'll upload it on request if you can't find it.

I've snooped with resedit and the like but I haven't found where

Thats because on this Install floppy, the personalization is written to the the data fork and not to the resource fork. You need a Hex Editor to edit the data, such as HexEdit and not ResEdit.

On a copy of the "Install" disk, open the file "Word Installer" in a hex editor.
In a hex editors window you should see a bunch of character rows in the hex column beginning:

0000000: 4C 41 78 etc, etc.

Remove all of the characters back to up to the 78 then add 0D (zero + D). The row should be:

0000000: 4C 41 78 0D

To the right of this in the text column you should see only LAx. (with the trailing ".").

Save and test, preferably using a copy of the modified install disk. It should ask for your user details here. It also will want to write them to the "Word Installer" file so it will insist on the disk being unlocked so it can do so.

Well, the user info can be stored anywhere including a resource of the resource fork of a classic Mac file, just happens this one writes it to the data fork.

I own the original install set of Word 5.1 and had when installing, used a back up of the install set. So still had the pristine original set and could easily check the differences that had been made to the Install disk.

PersonalizeWord_1.0.sit is not here
(I couldn´t find it)

It would be nice to have it at MacGarden since Word 5.1 is a popular program.

I have uploaded PersonalizeWord 1.0 here:
http://macintoshgarden.org/apps/microsoft-word

Thanks for the upload.