I have an '88 Westy with autotrans.  I mounted the same type of cup
holders to the sides of the box.  I had had to replace the lights in
the box so that I could see the autotrans selection letters in the
dark before, so I had learned that you can easily pop the top of the
box off to see what kind of clearance you have where you're drilling
holes.

I popped the top of the box off by just using a stiff flat bladed
putty knife - real simple.

John
Charlotte NC
88 Westy