Well ok - welcome,
First things first - someone else is bound to say it so I'll get this out of the way
Please don't EVER refer to a real Pontiac engine as either "big block" or "small block" --- there's no such thing. All Pontiac engine blocks from the "little" 326 up through the "mighty" 455 are all exactly the same size. Sit a 455 on your garage floor next to a 326 and the only way you'll be able to tell which is which will be by reading and interpreting the various codes stamped into the metal.
You get one free pass because you're a new ls guy and didn't know any better
There are other Pontiac die hards out there who wouldn't be so kind
On building a Judge clone. I'm a big fan of "it's your money, spend it how you like". You've got one big problem starting with a 68: The Judge started in 69, not 68, and visually there are some significant differences some of which might be harder to deal with than others. The biggest is probably the doors. 68 was the last year for vent windows - they were gone, replaced by one-piece door glass, in 69. Whether or not 68 doors can be outfitted with 69 internals and glass, I don't know for sure, but that will probably be expensive. Another major difference is the rear bumper, taillights, and tail panel where the taillights mount. All of those items are expensive and also I think only available as reproduction pieces.
Another difference is the dash/dash pad - completely different between 68 and 69, as is the console and shifter if it's an automatic.
Some of the things you mentioned, like the spoiler and front end are actually pretty easy. Not all Judges had hideaway headlights - in fact most probably didn't. So you'd be cool there -- but you'd need the 69 grilles which were different from 68. Also the lower valance and parking/side lights are different between 68 and 69. You'd need to lose the Pontiac emblem on the nose of the Endura bumper, that was another 68-only feature.
Most of the Judge 'visuals' needed to clone a 69 GTO into a 69 Judge are readily available because that's a popular thing to do. Making a 68 GTO/Lemans look like a 69 GTO is where all the hard part is. A lot of the work to make this Lemans look like a 68 GTO has already been done, so you're in luck there. It really would depend on how accurate/convincing you wanted to be.
There's another option: Take the car as is (which looks very nice in the photos by the way), and instead sink all the clone money into building a very serious stroker motor out of the 400 that's in it. You'll have a 'sleeper' that will flat out MURDER any stock Judge ever built
Your choice - just have fun with it.