If you have an editable mesh, which I assume you do if you are kitbashing, then select one of the multiple entries, such as ID 14 in your example above.
Go into the editable mesh tab, and select attach.
Then go and carefully select the areas sharing the same texture map, 18 and 55.
When they are all selected, then you must clear the attach cursor by clicking on another tab, such as the general shapes tab( Where Box, Cone, Sphere, etc. are located), and then deselect the mesh by clicking somewhere in the gray field it sits in back in one of the four working windows(top, front, left, Perspective).
Now, those 3 individual parts are one mesh. Move on to ID 6,and attach 57 and 65 ... rinse, repeat.
I'm sorry if this seems confusing ... I'm not much of a teacher, but I have the up most respect for most kitbashers. In the absence of modelers, some of you have become modelers, but even more important, you have been keeping this game alive with wondrous variety.
Thanks guys. I still have a place to come back to and model sh*t to blow up!
JM.