If you are going to drill the bars, you've got to be careful. First, the wiring for the switches is 50 years old and can be brittle. You can always lube it with something to make it slide in and through better, and I tie something like fishing line to the end to route that through the bars first to make it easier. You've also got to file down the inner and outer edges of the bars to make sure that when you route the wiring it doesn't cut the housing.
I've done both -- threading and notching the switch housing to route the wires externally. The latter is much easier, the former looks better...