The current photo suggests you are close and the guide is far too high on the downtube. There is a small foot on the guide that should actually rest on the BB shell. Please take a photo of the cable with the guide properly positioned, as given the current evidence it is difficult to believe that that is not the solution to your problem.

However, I have drilled and threaded holes in bottom bracket shells to install cable guides in the past. This is a solution for a rider, but if you seek to keep this in its unaltered classic form I agree other solutions must be found.