You actually don't need the ultimate doom Iwad to run TUCQ. What I do, and have done before I heard about the Fix of renaming chex.wad to doom.wad is this:
1.)Make sure you have cquestp2b2.wad in your legacy folder, but don't worry about it for now. Start Legacy Launcher.
2.) Run Chex.wad through legacy launcher (Depending on which version you have you might need to rename it to doom.wad)
3.) When it starts, choose a new game and choose your dificulty level. You should start on E1M1 of the original chex quest.
4.) In the console, type this: ADDFILE CQUESTP2B2.WAD
The outcome looks like this:

Don't fear, as this is normal, all you have to do now is type this in the console:
MAP E1M1
And you're good to go ^_^
If that still doen't work, come talk to me in my chat room on my site sometime or...im me or something.