A833 Rebuild help needed.

Ok first off, the snapring on the front bearing is not standard; you cannot use the one that comes on a new bearing, the retainer will not seal, and you may break it when you tighten the screws.. So you have to use the original factory snapring. But that has nothing to do with your problem.

Your problem is caused by one of three things.
1) one or more of the input rollers have fallen into the cavity. or
2) one of the two front brass rings is not indexed to the struts. or
3) the wrong tail, or tail-bearing is being used.
4) Edit by @inertia, see post #3
5) edit by AJ; see post #8; the 3-4 hub is on backwards.

looking at the pictures, my guess is the third gear brass is off the struts, but that is just a guess.

But there seems to be something else going on in the pics that I can't quite put my finger on; which is; the 3-4 slider seems to be offset towards the front, leaving the clutch-teeth on third gear way too far from the slider.

FYI
I never try to install the loaded rear end with the cluster pin installed.
Instead, I leave the cluster laying in the bottom as far down as it goes, then stand the case on end, with the input gear hanging down thru a hole in the bench, I made especially for it. and NO front retainer on it either.
Then with one hand,I drop the loaded rear end into the box clocked about 180 degrees out (see note 1), working the 3-4 slider around the cluster with the other hand. In my case I have the 4th gear brass already on the input gear, so I have to steer the struts into the proper slots.
With the rear end now sitting on the gasket, I rotate it slightly to expose the pin-hole, then slide the cluster into engagement, and drop the pin in. If the front washer has moved, and the pin will not drop through, then install one bolt into the only hole that lines up, and snug it down. Now you can lift the trans out of the hole in the bench, invert it, and align the washer, then finish sliding the pin in. Immediately I put the trans back in the hole, tail up, and finish installing the pin. Then remove the bolt ,rotate the tail into final position, and bolt it all together; finishing with the front retainer last.

Note 1
I have run into an occasional case where the clusterpin will not quite pass by the tail casting in the ~180* out clocking. So I always check before starting the assembly, and grind off the interfering casting.
Assembling the trans in this manner will take you only minutes, and with no stress.

If you still have issues;
Either; drop the input gear, or remove the rollers; AND remove the 4th gear brass; then test fit the rear. Your rear bearing install looks correct, assuming the rearmost snap ring is installed.
Easy-peasy Good luck

BTW-2
The rear bearing is not a standard bearing but is specific to this application. It is a wider bearing. So when you installed your rear bearing and fully seated it on the mainshaft, the rearmost snapring should have just barely fit,in the groove with ZERO space between it and the seated bearing..
If you had space, then you have the wrong bearing.
If you pushed the bearing back against the snapring, then the mainshaft would end up too far ahead.
If you left the too-narrow bearing seated, with space to the snapring, then there is only the press-fit between the bearing and the shaft to keep it seated. Once the trans gets up to temp, I cannot say if or how long the bearing will stay where you put it. Jus thought that was worth mentioning. You don't have to take it apart to look; the too-narrow bearing, together with the loaded mainshaft, will slide fore and aft in the tailhouse bore. And you might be able to see the error, thru the speed-O hole.