I drilled my 250 six cylinder crank while the engine was out of the truck. The 250 in a '37 chassis doesn't leave enough room to do much work on it, yours results may vary.
Drilling isn't all that hard, start with small bits and work your way up to the correct size for a small black Chevy bolt. Keep clearing chips out as you go. You will need to go pretty deep unless you purchase a two taps, one a standard taper tap, and one bottoming tap, but you still need go past the total length of the bolt.
Being centered is best along with straight, buts its not the end of the world if you get off sideways a little. With a tall drill press, it takes no time to drill out the hole, I did mine still in the block with blocks of wood to support the engine. Be sure to use new bits and taps, it will go much easier.
On a side note, DO NOT USE THE BOLT TO INSTALL THE BALANCER ! Instead, install a piece of threaded rod, something harder then ALL-THREAD, and a few large greasy washers and a long coupler nut. The greasy washers help with friction, the long coupler nut has way more surface area to grip more threads under torque. Once you have the balancer in place, unthread the rod, clean out the hole, and Loc-tite the bolt in place with a torque wrench.
The optimal way to install a balancer is with heat and cold, Crankshaft in the cold ( today would be a good day 18* F out ) and balancer in an oven at 200* F . Once both have heat soaked for an hour or two, in one quick move, shove the balancer on. Most likely it will go 100% of the way, if not, pull it the rest of the way or block the rear of the crank from moving and drive it on with a block of wood and large hammer.
