You should be able to use your 45/64's straight drill and then carefully run your pipe tap in.
Tapping cast iron is not too hard, keep it as straight as you can and when it gets hard to turn, back it off, clean and then back in 1/4 to 1/2 half turn deeper.
Use some kind of cutting oil also. Cooking oil will do in a pinch.
Rule of thumb is tap till you have about four threads still showing on the tap.
Be careful, it is real easy to go too deep and then your fitting will not seal.
Another trick is to magnetize the tap so that most of the metal chips stay on it.
Rake the shank of the tap over a good magnet and not the cutting edges.
Clean the tap after every half turn once it's starts getting hard to turn and then gage the depth with your fitting.
All fittings are also not all the same thread length, so you'll have to go slow until the feel is right and where you want it.
I also prefer pipe dope over teflon tape.
If you lived closer, I'd come over and show you how to do it.
Hope this helps and let us know how it worked out.
Don
