I believe it has changed somewhat over the years. The barrels are press fit into the receiver then secured with a pin. The older models are splined with one double wide spline to properly locate the barrel. I believe the newer ones are just smooth which makes alignment a little more difficult. It's not a job you can do quickly and easily in the field.
The end mill bit will make it much easier to get the hole started on line. After you get the hole started in the barrel, you can switch to a regular 1/8" bit to finish the hole. If you don't have an end mill, the way you have to drill across the edge of the barrel will cause the bit to try to wander off.
This is assuming you are starting with a new barrel which comes without the groove for the pin, as I was. If you buy a used barrel, you'll have to hope the holes and groove line up. They probably will, but I just don't know. Marlin obviously drills the hole/groove into the barrel after it is installed into the receiver, so it's custom fit in each gun.