Clutch question before my first time moving D6-9u

Clutch brake should stop the aft portion of shaft between the flywheel & the transmission, The Clutch Disc assembly seperates this drive train.
With your description of it "bogging down" the main engine when you push the clutch lever forward, the clutch brake is trying to stop the engine.

I suspect you have a stuck clutch plate, either to the flywheel or the pressure plate half, maybe both.
Not being able to engage the clutch could present a secondary issue, maybe someone tried to adjust a worn clutch and with the possible buildup of rust and delaminated disk material, it has swolled beyond limits.

CTS

TY @Ctsnowfigher, As I can see the disengaged rear clutch plate has a nice gap. It would have to be the front plate I assume

UPDATE,
I got the clutch to "click" backward and the lock engaged. Just had to pull much harder than I would have thought.

I am going to publish a video of the un-slowing clutch.

Can you, with a helper, put it in gear, and then turn the main engine over with the pony motor to see if the clutch can actually be stopped? The problem could either be that the clutch is stuck as cts wrote in which case the pony will move the tractor, or it could be that the clutch brake is inadequate, in which case the pony will not move the tractor. The brake could be inadequate because the lever is hitting the end of the slot before full brake engagement and that could be due to the brake linings being worn out. If all the linkage is worn, the total lost motion could mean that it's just not adjustable to spec. There are adjustments that can be made for the clutch, the clutch brake, and the lever. These all have to be set correctly for the whole contraption to work as designed. If you have the serviceman's reference, the instructions are in there. If not, if you can access it in the library here

Great idea. I will give that a shot tomorrow to determine if it's truly stuck. I also realized today I have a d6 5r 4xx series. Lucky I have the serviceman, parts and manual books. 

https://youtu.be/r6JsjHUOby8

Here is a attempt at recording the issue.

A Parts Book, Servicemen's manual and an Operation and Maintenance Manual specifically made for your Serial Number should be one of the first purchases after buying the machine. Cat uses the S/N to keep track

Starting Engine -
Remember these engines have a very limited cooling system. They depend upon the main engine water pump and tend to heat up if run for extended times.
If you do attempt to move the machine with the starting engine - ensure - compression release is in released postion and fuel to main is OFF. Last thing you want is for the main to fire up. 

***  Another note of caution - those clutches on the starting engines are not large and not meant to take the load of the entire tractor, they are engaged when engine is disconnected from the transmission and compression release is open -  

If you do have a disc stuck, sometimes you can seperate them with a tapered object, much like a screwdriver, driven between the fiber and the metal portions - little at a time in mulitple places around the diameter. Careful you do not overdue as you can damage the components. You want a wide tool - to evenly apply pressure, too narrow and you risk breaking the disc.

Sometimes one can get "lucky" and use both steering frictions (act as clutches which they are) to put machine in gear, with the main clutch disengaged, and machine in a higher gear, best at low rpm on the engine, release the steering frictions in unison while applying both brakes. Just keep in mind if it does not release, you must use the frictions in unison to stop and holding them back, take machine out of gear. This puts a load on the clutch and with it in disengaged position it make break the lining free from the pressure plate/ flywheel.

CTS

My next idear would be point it in a safe direction. start engine and warm it up a bit then stop engine ,put it in third or fourth gear, remember forward / reverse is lever back for forward and push start it with another tractor and drive it around with the clutch disengaged might take a while but it should release.

Thank you all for your advice. I used a prybar to pull the thick 1 inch flywheel (?) away from the rear pressure plate (unsticking it) and greased everything up and it freed up. I have a running functional bulldozer now. Thanks, everyone again.