The Small Block Chevy balancer bolt is a 7/16 X 20 thread and the correct drill is 25/64ths. I drill my pilot holes about 9/16" deep so I don't get into the oil passages in the crank.
By drilling/tapping you can use a standard Harmonic balancer installation tool to press/draw the balancer home. I would not recommend trying to use the balancer bolt itself and torque to force the balancer home. You just don't have enough threads to take the force. A balancer installation set is available from AutoZone and other FLAPS. You place a deposit on the tool and get a 100% refund when you return it.
The installation kit contains a couple bearings...the center stud screws into the threads you cut for a purchase....and then you spin the nut down against the bearings. The center stud doesn't turn during the process so you won't have to worry about boogering up the threads you cut. When you get the balancer with the loose (seal centered) timing cover, I'd start the timing cover bolts in the block and gradually tighten them as you seated the balancer. I like to use rvt (sparingly) to glue the gasket to the timing cover and then after the rvt has cured, use #2 Permatex where it seals against the block.
On the 235 block, the 2 bottom timing cover bolts are removed/installed with the pan removed to get the cover on or off. This is a real hassle and if your engine is torn down, it's an easy fix to thread the front main journal with 3/8" threads and drill out the holes in the block and cover so you can just install all the timing cover bolts from the front of the block. Should you ever need to pull the timing cover it eliminates having to drop the oil pan.
If you have questions send me a PM.
Dave
