Custom terrain import problem of non power-of-2 image, please advise if possible.

I have a fully custom terrain that i would like to import into Unreal Engine 5.4.1.
The terrain is NOT a power-of-2 image and it MUST stay that way in order for the remaining of the assets and textures to match up.
It’s final resolution is of 38913x22529 because each tile of 257x257 overlaps by 1 pixel once merged/stitched together, total tiles are 13,376.
While attempting to import the r16 tile sets in to UE the size is wrong, obviously UE expect a perfectly square image.
For the tile sets i get the following results once selected for import:

While for the single terrain image the main resolution is correct but final import is wrong:

But even so only partial terrain shows, while the remaining terrain is purely flat, no Z scale/depth.

If i try to calculate the X/Y scale is impossible since UE is hard-coded to work with square and if i change the values to X the Y will use same values which is wrong in this custom terrain.

Is there anyway at all i can use my custom terrain to import as is, no adjustments to the terrain, while also preserving their albedo/normal/masks/etc which those are indeed power-of-2 but NOT square either.
If possible, in my example what would be the correct scale values for X, Y and Z respectively?

I should also specify I am new in UE, and trying to find best way to import such large map in UE and how to achieve that in order for it to be streamed properly once all objects/assets have been placed on the map, the way once you advance it loads one and unloads the previous one, etc.

Looking forward for some pointers from those far more experienced than myself in order to achieve this.

Would anyone at all happen to have an idea, solution, etc?

Anyone has a clue what so ever as to why after import, every time no matter what the region seams are visible?