[![enter image description here][1]][1]

 The _Map Range Node_ and _Attribute Statistics Node_ are the key nodes. The Map range is used to fit values within the desired range, while the Attribute statistics will let you know a set of values from the given field.
 The _Min_ and _Max_ values in the _From_ sections on the Map Range Node should come from the min and max values from the attribute statistics (Remember to set it to Instances). I arrayed them in the Y-axis, so I separated the position and only edited the y values. The _To Min_ value should be half the cube's size. This is because the cube is instanced from the center, while the goal is to start from the exact beginning. So we add the other half to accomplish the full size of the cube. Since we also want the end of the last cube to be at the exact end of the line, we simply subtract half its size from the length of the curve to do the opposite of what we did to the first cube. This becomes the _To max_ value. I recombined the vector with edits only on the Y-Axis and used it for the _Set Position Node_. I hope this makes sense.

Here is the node setup:

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/CDCII.png
  [2]: https://i.sstatic.net/GOGxx.png