If you're exporting terrain that is mostly displaced in Y only, then just use the heightfield method.
Make it large, like 8k large or higher.
A method that doesn't use the built in heightfield nodes... [do try Terraman too btw]
If you get a Get Altitude node, plug that into the luminosity of a default shader and put that on your landscape, with your last displacement node going into Child, with diffuse, reflection etc all at zero, render it top down with an orthographic camera, then that results in a black and white altitude map which when loaded as a push map in your 3D software should result in correct altitudes, providing you make your grid the same width as the orthographic width of your camera. Don't have lights on and set the ortho camera to have exposure 1.
It might seem like legwork but it works perfectly every time and avoids all this terrain export, triangulated stuff which doesn't guarantee nice shading in your renderer. Then also you can project your colour texture the same way, rendered from the same camera. Things will line up nicely.