I will not waste any further of my time debating this, but the Dart SHP block (and I believe almost all aftermarket iron blocks) has every single accessory hole drilled and is marketed as, and I quote
Dart's new Special High Performance (SHP) small-block meets the need for an affordable, precision-machined cast-iron block with superior features. Designed for high-performance and heavy-duty applications producing up to 600 horsepower, the SHP block is the ideal starting point for hot rodders, drag racers, circle track competitors, off-roaders, and high-performance marine enthusiasts.
Not performance only, but ideal for everything from hotrodders to marine applications. It's not 'race only' and your assumption thats why they are not drilled is plain incorrect. The fact is they are not needed unless your car will see exrended periods of under 1500 rpm in the heat, seriously. Thats why the engineers built them that way. And since my answer was based on the OP's original question, that does not apply.
A good reason? How about, because you don't need to in even a mild performance application
? Again as I have said I know many stories about guys drilling the heads incorrectly, so I figure the fact they are not needed, they are subject to screwing up, and are an additional cost if a shop does it, thats reason enough for me. Plus back when I worked as a mechanic the reason the 400 had a bad rep was heads cracked at the steam holes
. If you want to drill holes in your heads, by all means have at it, it's not going to hurt my feelings.