For that file you could add it just about anywhere. Just look for similar format for the line.
And just stick it in.. So where you see "q!upload" as a line just stick it in there just below the last one but before the closing statement and you may want to add a comment so you know what it is for in the future.

And don't forget.
Backup, then backup and then backup again when making changes

I just think the album would be safer for you. It is just a little difficult to set all the paths. As I recall some are absolute and some are just urls and it was not clear which was what. Besides it really works stand alone from classic so if you hose it up it won't break your board.

Also I don't know how you value your time. But for $99.00 You can renew your membership access here and just upgrade to the latest threads.
Assuming you have the requirements PHP,Mysql on your host.

That has a gallery and users can upload files & everything?

Also, if you would refresh my memory, when I upgrade do I have to set everything up again? What does it entail?

Would you know off-hand where cp2_forumedit.pl & cp2_forums2.cgi are located? They're not in the cgi-bin like everything else.

Okay,
This site has the latest greatest version. If you visit the gallery forums or test forums you can see and or test some of the features. And goto your mystuff menu at the top of the page and check out the preferences and profiles settings.
Also some features available are not turned on here. But most are.

It will not be much different to hack up your board than just upgrading. You just need to be sure you have the minimum server requirements at your host. Most all have php. But the most critical is MYSQL. Otherwise threads will not work at all without it.

So here are the steps.
Renew your license which is the pricing & order. Select renewal.
You will need to have your license info.
Pay $99.00
This gives you access to the members area for one year.
In this page you have access to the latest threads software and the last classic version and some importers from classic to threads which you will need.
You can ask the question but I think since you are using classic 6.x you don't really need to upgrade to classic 6.7.2 before upgrading to threads 7.5.3
Then the upgrade begins.
You can pay someone to upgrade for you be it UBB,or someone else that may solicit you here. Or do it yourself.

You will then upload the new files, set file permissions, setup your mysql database,run the importer script and then go through the control panel to input your settings.
I know it sounds like a lot but it really is simple. Especially if you have done it once.

It also gets easier when not upgrading from classic. So if a new version comes up. It is just a simple matter of running upgrade.php

cp2_forumedit.pl is in the noncgi/templates folder
cp2_forums2.cgi not sure there is a cp2_forums.pl in the noncgi/templates folder.
That might be it.
I am looking at v6.7.2 and like I said before some files have changed. So you would open the file and see if you have similar content to find and replace.

You mean ubb/templates?

And with cp2_forums2.cgi, I've got cp2_forums.cgi but not forums2. I don't suppose I could just skip this and hope everything will work out.

Is every file that has .cgi extension going to be in the cgi-bin?

well the default folder is noncgi/templates but your install may have used a different structure.
As far as forums2 file. No you can't skip it. Assuming you are using the beta version. It could have been a file name change on the version you are using. Such as the extension went from .cgi to pl or vice-versa. Or the file name changed. You need to search for the code string you are replacing or even a partial search because if your version is newer the string may not be quite the same.
Like I said it is a chore.

And I would not rely on all .cgi files being in the same folder. And the same goes for .pl or other extensions

How do I determine if I'm using beta...I'm almost sure it's not, it's just 6.7.1

The strange thing is I downloaded an updated version of the mod which was designed to work with 6.7.3 (it says it works on 6.7.x in the instructions). Both versions refer to forums2.

How would I go about doing a search for code string?

Okay I don't wish to sound curt but you really need to start posting at UBBDev.
Searching for a string is simply using a search across all of your files for the subject text that you are looking for to replace or add a insert in a file.
This means not having a file open and search for a pattern of text but to search a whole folder for the text pattern.
So for me I use a utility called grep for windows. Because windows search does not work for files with double extensions such as mysql.inc.php.