This is the Dart cast-iron 8.2-inch low-deck block (PN 313847) with four-bolt main caps, a priority main oiling system, and siamesed cylinder bores. This block is based on the old Boss 302 with an 8.2-inch deck, 4-inch bores, and 2.249-inch main journals. The block is sonic-checked and treated to a 0.250-inch overbore (that's 1/4 inch!), bringing bore size to a whopping 4.250 inches. With a stock bore spacing of 4.380 inches, this leaves a scant 0.130 inch between cylinders. But not to worry, Chevrolet buffs have been doing this with big-blocks for years.