Transmission is stuck in first gear?

Thanks for the image. When the engine is off the gear shifter position does not matter. It should move into first gear anyway, is the transmission full of fluid? Can you install the shifter and try to move the gear shifter back into neutral? If not, something broken inside the transmission, and it will need to be disassembled and repaired. Here are exploded diagrams of the transmission so you can see inside. You can upload a short video of the problem so I can confirm the repair as well. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem.

Normally you shift it into 3rd or 4th gear before pulling the shifter. What you should be able to do is to use a screwdriver and reach into the shifter rail box and pry the rails back into neutral. You may need to rock the truck a bit to remove the tension inside the transmission and hold the clutch down to move the shaft. The other option is to drop the transmission out and split the case and align the rails that way if the shift rails don't move with a screwdriver. What can happen in them is the shaft can overextend inside the transmission and then the shift forks can bind. Usually though you can pry them into position.

Low fluid in that transmission shouldn't cause it to stay stuck in a gear if it wasn't a problem before the clutch replacement. The normal issue is that you pull the shifter out then as you jockey the transmission around you jolt it or tip the tail down and the shifter rail moves and binds. Then the shift forks on the other end of the rail bind up and you reinstall the shift lever on top and now it's bound up so you cannot move it into neutral or other gears.
If you remove the shift lever again and look at your picture the yoke (part 11 that looks like it has the ears and screwdriver slots on each side) is supposed to rock from side to side while in neutral. The socket 10 that the shift lever goes into on the other hand slides front to back. When they bind it's normally because one is trying to slide out of say first gear, but the yoke is setting in the middle like it's in neutral. That appears to be the way yours is at the moment. So, you need to move both to get it back into neutral. If you cannot get it back into neutral, then you will need to split it and look at the 3 shift forks and see if one of those tipped the wrong way.