I just remembered today having drilled a series of drain-back holes in the '65 273 block that I ran in my bracket racer. They would have been placed between each pair of lifters in that little spot where a small amount of oil tends to sit and puddle. I also had painted the interior of the block with Rust-Oleum rusty metal red primer after the mods. Now, I painted up the 340 block I am preparing (see photo), but this time I am going to forego the drain back holes. That was a little scary back in the '70's, I always wondered if I was weakening the center structure of the block (the holes were not very large). I don't believe I thought of the idea of drilling the holes on my own, I must have read about it somewhere back then in the 1970's. Has anyone found a reference book or article that discusses this procedure? Anyone have photos of a block modified in this manner? I was just curious what was out there. Thank you.