Drilling is recommended, you can even buy a bolt and washer kit that includes a 7/16NF bolt with the correct size and thickness washer. I made my own from a piece of alloy bar. It's easiest done with the crank out of the engine, and a good machine shop should recommend that it be done when the crank is ground. If you're doing it yourself, buy the kit, drill the hole about 3/8" deeper than the bolt (minus the washer thickness) and tap. Torque is in the 90-100 ft-lb range, iirc. After everything is together (engine is ready to run) take the bolt out, clean the bolt and hole and re-torque with red loctite. Don't worry about the balance, just drill as close to the shaft center as you can get. Unless you paid extra, your crank and rods are out of balance far more than the weight of the bolt head and washer.
