Where backbone bolts are specified for our boats, they are all 5/16" diameter carriage bolts. Standard jobber's drills are seldom more than 4" long, and most of the bolt holes are longer than that. That leaves you with a problem...either layout the bolt holes and carefully bore from both sides so that the holes meet in the middle, which is possible, or spare yourself the trepidation and simply get a longer bit. That's where these 12" aircraft bits come in. Not only will these reach all of the way through in a single pass (be sure to clear chips as you drill so they don't jam), but they are easier to line up so that they emerge where intended on the opposite side. These are American made, and they aren't inexpensive, but they will save you time and frustration and they will last a lifetime.
Another place a long 5/16" bit is called for is when using drifts to reinforce a transom. That portion of this Christmas Wherry transom above the level of the quarterknees is weekened by the sculling cutout. Adding a 10" long drift port and starboard insures that a mishap won't break the upper portion of the transom. Drift reinforcement is also a good idea wherever you are going to be hanging an outboard, where the thrust of the engine puts a considerable load on the upper portion of the transom.