I have a 1965 409 Block, standard bore, casting NO. 3857656, with an "X" cast in the timing chain area. At the top outer edge of each cylinder, there is a notch, about 2 3/4" long. Now, I have 2 books, one is "Cars and Parts" Chevy V-8 Casting Numbers, this book says with the "X" and this casting no.,that this is a passenger car block. My other book, "MuscleCar and Hi-Po Engines, also shows several pictures of Pass. 409's with this notch, and they say the notch is for exhaust valve clearance. But, I am confused, because several knowledgable members of this forum state that this notch means you have a Truck Block. Were there 2 different sized notches?? I'm trying to learn. Also, this block came with GM rods, and 11 to 1 pistons. What I'd really like to know is what my actual compression ratio will be, IF, this in fact a truck block. If it's a car block, then I am guessing comp. ratio will be almost 11 to 1 with a single head gasket. Any valuable info would be greatly appreciated, from some of you experts on this engine! Thanks!!!!!! Brian/Andrew.