From your image it is unclear what is going on, as we can't tell which part of the model we see in the uvEditor, and even then the resolution is too low. First thing to try would be to apply Unfold several times, it usually straightens and evens out the texel distribution based on 3D proportions. If it doesn't help, then, since you are using SmoothPolies, that demands specific modeling approach, and here it is unclear whether stretching happens because of insufficient amount of the edge loops near corners, or there are some unwelded edges.
If you still can't manage to fix it, attach a model, zipped MA, to your next post.
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Maya2019.1 @ Windows10 & GeForce GTX1080Ti
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