A833 assembly tricks

It shouldn't take more than a few seconds to drop that tail in there. I think you are doin' it the hard way.

Here's how I do it;
I leave the loaded cluster in the bottom, with no pin in it.
Leave the front retainer off and push the input gear down as far as it goes, into the hole in my bench that is there just for this purpose.
Install a spare yoke in the tail, to make it easy to turn it.
Orient the your loaded tail upside down to the main case and drop her straight down, turning the M/S to engage the 4th gear struts in the brass .
Orient the tail such that the pinhole is fully accessible
Pull the cluster back into alignment, and install the pin
Raise the tail slightly to not tear the gasket, and rotate it into proper alignment
Install 2 bolts finger tight.
Lay the trans on it it's side. make sure the 4th gear brass is indexed, then push the input gear into the case.
Install the retainer and finish up.

Note-1
sometimes the cluster pin will not pass by the upside down tail. I always test fit the empty tail to the empty case prior, and clearance the tail as may be required.
Note-2
Be careful not to drop the 3-4 slider; it's a PITA to re install it; I always spread ALL the strut springs before assy,which seems to make enough friction, that for me this is not a thing....... anymore.
note-3
As the output shaft enters the input, and the loaded tail comes down, the struts have to be aligned with the cut-outs in the brass before the tail lands. For me, I am strong enough to do this simultaneously. You may wish to install 1/8 inch spacers on the back of the case prior and land the tail on those, then go align the input, then spit the shims out.
Note-4
Some guys load a mock pin into the cluster in an attempt to not allow the needle-rollers to fall out. Well yeah, if yur using Vasoline I guess, lol. But I use a light wheel-bearing grease so have never in hundreds and hundreds of assemblies, ever had a roller go AWOL. Same grease is used in the input.
That's how I do it.

Now;
did you deglaze the cones?
Prefit each brass ring and make sure they bite?
repoint the clutching teeth?
Back-cut the clutch teeth as may be required?
Reset the cluster end-play?
Verify the fork travel and the reverse lock-out, and make sure that it is impossible to engage more than one forward gear at a time?
Additionally;
The A833 suffers from the nuts coming loose on the external shift levers. When this happens, the lever-slot to internal-lever interface takes a ****-kicking, and they get sloppy. Once sloppy it will be impossible to keep it from happening over and over. Loc-titing the nuts is NOT the complete answer.
What you need to do is loc-tite the interface. But you gotta do it in a way that will not allow the loc-tite to migrate into the cover between it and the pin.
So before the cover goes on, I pop the internal levers out and grease the tubes, reinstall the levers, then go clean up the pushed-out lube from the outsides. Next clean everything that is to be loc-tited.
Next install the external levers in their correct orientations, onto the studs.
Next fill the interfaces with loc-tite (I use Blue), and loc-tite the nuts, and torque 'em up. Wipe off the excess, invert the cover, and walk away. Come back every several minutes and verify that you can still shift it, proving that the loc-tite has Not, by capillary action, gone up the stud into the cover. I panicked one time when using Loc-tite Red; but a lil more force broke it free. I switched to blue after that. No more problems.
With the levers now immobilized, your Neutral gate will stay where you put it, for years, perhaps decades.
Do the same thing with the shifter adapter-plate at the back; blue loc-tite the three conical-head bolts. But you may wish to do this after the trans is up in the tunnel. Your choice.