When I made the WAR FW-190 mount, I found it easier to add sketch planes instead of 3D-sketches. You will have to define points where all tubes meet, define planes through the points and draw sketches with lines on these planes. Then you can use Weldments to place and trim tubes. You will probably have to define seamless tube sections if you use weldments.
Name:  Capture.jpg
Views: 1932
Size:  7.8 KB