The by-pass is a 292 thing. You will have to switch to a bolt "up" starter. One hole is already there. You have to drill and thread the other. There is a template and instructions in a book called "Chevrolet Inline Six" by Leo Santucci. This will answer all your questions and more. The block cracking on the first bolt hole is a "250 thing" and is mentioned in his book.
My two-groove pulley looks very much like a two-groove 250 pulley but the timing mark location is different. The 3-groove are from big trucks with pumps for air brakes.
Correction
I was thinking only about my block.
You only have to drill and tap if you will be using a offset mounted starter and large (168 tooth) flywheel. Santuccis' book says 1973 and later blocks are drilled for both starters.
