Package arrived in the mail!
In fact the slider is a little bit longer than the one it is replacing, however the final verdict is that with a little bit of elbow grease, it works just fine!
After swapping out the (perfectly compatible) wiper with the one from the MT-400, I also swapped the retainer with the stabilizing legs into the bottom of the new pot. Really illustrates the size difference. My thinking was to put the stabilizing legs at the bottom of the pot, where the cutoff is set to zero. This way when the wiper runs to the top of the slider, the post hits the case just before reaching the top of the slider. However since there is also a “sustain” knob which boosts the top of the cutoff higher, I figure that the loss here is less great than if you were to lose the ability to put the slider all the way to 0. When I had the filter fully open, switching between filtered and non-filtered sound had a very slight, what I would call trivial difference.
To compensate for the length of the new part, I stripped some solid-core wire and bent it around the top leg of the slider. Then bent it so that it would fit the footprint of the original slider.
Though this is technically a compromise and not a 100% perfect replacement, I personally am 100% happy with this fix as I believe it to be close enough (someone operating it fully closed up without looking at the guts inside would have no idea that a repair or modification has been made).
Thank you for your suggestion and hopefully this can help someone out in the future!
NOTE: I originally made this post with inline images but the forum wouldn’t let me post it due to being too new a member for that many pictures/links. Here is a link to the gallery instead: