"The frame is a late 90s Bianchi, so it should be 68mm."
"Should be" is not the same as "is."
A careful measurement to 100% confirm this will take little time or effort and might save you a lot of grief.
Kano Kroil is the top penetrating oil in my book, it might be worthwhile to get some and try it plus a careful application of heat/cold.
I suspect that drilling into a bottom bracket is a lot more difficult than it sounds, especially if it is Ti like Kimmo suggests. Getting it done without breaking a lot of tools or having them walk sideways into the threads will take luck as well as skill, patience and a careful setup.