Do You Know That You Can Extend Using ClipCurve Brush

Extend & flatten the top or bottom Edge with ClipCurve brush.

Before that, mask either the bottom or top edge, whichever you want to lock.

In this case, the top.

In this case, the top. I want to extend the bottom edge.

Use BZM.

Zmodeler brush.

Hover over a Top edge and press Spacebar.

Click on Mask > Edgeloop complete to select what actions I want to do.

Click on a top edge to mask the whole top edge loop.

Ctrl Shift Click and Drag from left to right below the polyloop strip to test it.

Yeap, it works!

Now I can look at the reference image straight on, extend the polyloop accordingly.

Ctrl click and drag on the background to Unmask everything.

Clip the top part to the height as shown on the image.