Just rebuilt my 23 spline 4-speed for the Cuda. The problem is when I move the 1-2 slider the strut keys pop outward. I had to pull the trans due to this as it stuck in 2nd gear. What gives?

Did you reuse both the retaining spring wires inside under those keys on both the front and rear of the keys? If not you should
I stagger them so both of the ends are not aligned under of the slots in the hub also

Cab, I did not replace the wires but they seemed ok. I did stagger them also so that the tabs pop into different keys from one another.
All three keys pop outward when I move the slider all the way into 2nd.

Did you replace any parts of the synchronizer assembly? There a couple different versions. Also the hub has different length shoulders, is it backwards?

I don’t think it is backwards unless it was before. Here is a pic

If you do that with the side cover off that is a normal condition. The shift forks will not let it go that far when the side cover is installed. Now if it still does that with the side cover and shift forks installed we have a problem.

Gus

1-2 and 3-4 are different make sure you got them back right.

https://www.musclecardiy.com/transmissions/how-to-rebuild-chrysler-a883-4-speed-transmissions/#:~:text=The%20A833%20is%20a%20side-loaded%20transmission%20with%20all,The%20outboard%20design%20requires%20an%20additional%20mid%20plate.

Quick overhaul hints with pictures, as mentioned the two syncro hubs have different lengths. If you scroll down the pictures it shows the hub differences.
And how is the endplay of the unit? How much movement if any of the input and output shafts in and out? That can cause your trouble as well.

https://www.musclecardiy.com/transmissions/how-to-rebuild-chrysler-a883-4-speed-transmissions/

this perhaps ?

Now begins the debate over dextron or gear oil… good article. Tim

my one buddy had a chevy pickup years ago that smoked up a storm and used oil like it was fuel. he dumped used ATF and gear oil in that thing, plus whatever used hydraulic oil and motor oil that my brother and i could provide at the time. he ran that thing for years like that !
and these boneheads today think "rolling coal" is something new............

What 4gear said. You have the slide over too far and rubbing on the reverse idler. Does not move to the extremes with shift fork. The slider only engages those syncro teeth on the gear and not that deeply. That is why if the gear teeth are rounded the gear is bad and will probably pop out of gear.