Don't forget, when drilling thru the far side of each hole, all the chips created are going into the interior of the fork and you'll need to get them out somehow. When you have just a blade, not brazed to the crown yet, you can knock the chips out the open end.
I have done the thru-hole type BO on a finished fork at the customer's request, but it was a bit of a pain to get the chips out. I used the pencil-eraser-sized magnet on the end of my carbide scriber, it fits thru the 6 mm hole and collects the chips, but it's still not what I'd call easy. If you leave so much as a single chip in there, the fork will have a rattle. Pro tip: when you drill that second hole (far side of the blade), interrupt the cut frequently so you don't get long spiral chips that will be harder to pull out.
Duane's mention of rust preventative is a good point — if this fork has any petroleum type goop like Framesaver in it, you'll want to wash that out with solvent or else you'll have smoky flames coming out the hole while you're trying to braze it.
For a while there (late '80s early '90s maybe?) those racks that need the thru-type BO were popular with my customers, so I was putting them on a lot, but almost always before brazing blades to crown. My preference was two separate H2O top-hat BOs, with the diamond reinforcements mostly because they look nice. Then two separate screws, one from each side, rather than a thru-bolt and nut. Functionally the same, just an aesthetic preference.
Mark B