Yes, the draw tubes are about the same length. However, , the new, square base, focuser when racked down into the tube extends to nearly the secondary mirror. Won't that impede light to primary?. Thanks for the observation. I'm learning!
If the draw tube extends that far into the tube, then yes it will be in the light path. But, your original focuser is much taller, so my bet is the focal plane is much higher than the short new focuser at it's minimum height. As a guess, the focal plane is probably at least an inch higher than the original focuser at it's minimum height. So, the new focuser won't be completely focused downward as shown.
In fact, you'll very likely have to rack it outward to reach at least the minimum height of the original focuser. Then a little higher to accommodate the focal plane. You may be operating near the outward most focuser travel of your new focuser. So, I doubt the draw tube will intrude too deep into the scope. And, since the draw tubes are the same length, you may have enough focuser travel to reach the focal plane.
Is the new focuser racked all the way out in the images you posted earlier? They both looked racked outward a bit. If the old focuser on the left is at or higher than focus, it looks like the new focuser might be able to reach the focal plane, too. But, it might not have enough travel to go much further out, if it's not fully out already.
Edit: Wagging a guess. If the original focuser on the right is nearly racked in, and if the focal plane is about an inch above it, then I think your new focuser will probably rack upward about 2" to 2.5" to reach the focal plane. It may have about 1" of outward travel remaining. Now, you can fashion a block to raise the new focuser a little bit. You are addressing the different curvatures of the smaller tube, anyway. I mean, you can make it work.
Edited by Asbytec, 27 November 2021 - 03:06 AM.