for the mount, you DONT want the hole threaded in the plate... makes it very difficult to tighten and keep the camera straight as earlier said.
However, I do like my bolts to not fall out and get lost if the camera is dismounted. So you need to put some retainer to keep the bolt from falling out (if you care.. )
Me, I cheat... I tap the mounting plate hole like you did. Then I grind the threads off of the bolt that are closest to the HEAD of the bolt. Enough to allow the bolt to turn freely once its threaded through the hole. This way I can turn the bolt into the nut on the camera without worry. Bit more pain than its realy worth.. I also just keep lots of extra 1/4 X 20 bolts of various lengths. In fact, any hole, nut, or bolt that I need for my DIY projects is 1/4 X 20. Keeps it simple.