These economy short blocks use a stock production 351 block. I have seen them commonly make 600 flywheel horsepower. It will take some work to get 425 rwhp out of a stock cubic inch 351. It will take a good set of heads, matched camshaft and compression to get you there Naturally aspirated. Your best bet would be to settle on what heads you want to go with and contact comp cams and have them pick the right camshaft to meet your needs.