So far it looks pretty good. The only thing I see thats not the same as the original is the plasma pistol spawn. The hallway is shorter then a whole box length. Other then that, good job
You should try to make a switch similar to the one on pallet parade that makes the boxes move (like in elongation). You would have to use the movable blocks of course, but it would be a nice touch.
If I remember correctly, pallet parade relied on having pallets being constantly destroyed in one place and then them instant-respawning in another, forcing the pallets in front forward into the destruction area. If I am wrong, please post how it was done. The point is, you cannot destroy blocks in the way that you can destroy pallets. Good idea though, if you can get it to work.
If you had enough pallets then it could be done. Using what is available on Sandbox, your limiting you map to up to 8 pallets long (8 for each belt). It wouldn't really be worth adding the conveyor belts,
If the map is in the skybubble, you could have the crates drop through the grid and set to spawn instantly. Arrange spawn times to have them look good. That might work, but I don't suggest overlapping belts.
You wouldn't have nearly enough pallets. Plus, they would get destroyed and everything would get completely messed up.