Error when trying to combine two complex bodies from converted STL files

[Anonymous]

Hi, I'm trying to build a workflow in order to convert complex organic STL files into a STEP files.

I have managed to convert the whole model into solid bodies by splitting up the design. I had to do this because of Fusion 360 limitation of 50 K polygons when converting from Mesh to b-rep. Now I want to merge these bodies but I get error messages such as   

"Error: There was a problem combining geometry together.
If attempting a Join/Cut/Intersect, try to ensure that the bodies have a clear overlap (problems can occur where faces and edges are nearly coincident).
Failed to Boolean bodies together"!

I also get this error when I make the parts overlap.

 Is this operation too complex for Fusion or am I doing something wrong?

[TheCADWhisperer]

Can you Attach the original *.stl file here?

[Anonymous]

I rather send it to you privately as this is for a client and I don't want to publicize it on the web.

[Anonymous]

Here is 2 parts of the model. One version where they intersect and one version where they align.

https://www.dropbox.com/s/al1r5lghswdrbmx/intersectiontest_closehole_clean.zip?dl=0

https://www.dropbox.com/s/swl7z71k9eiqj1b/test_closehole_clean.zip?dl=0

I'm able to convert these to bodies but after that i cant combine/merge them.