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This is the 401 block we scored for the build. The primary differences between the 401 and the 360 are the bore sizes and factory-forged internals. Stock 360 blocks have 4.08-inch bores with cast 3.44-inch stroke cranks, and the 401s have 4.165 bores with forged 368-inch stroke cranks and thicker main webbing. Note the ubiquitous AMC oil mod that takes oil pressure from the main gallery and feeds it into the No. 4 and 5 main bearings. We've found from firsthand experience that this is only necessary if you plan to run the engine to 6,000 rpm all the time. If you have a stock Gremlin, focus on the oil pump cover and timing chain. They are bigger problems.