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I've got three 400 s.b.c.'s. Two of them have three freeze plugs and one has two freeze plugs. One of the three freeze plug blocks has 4-bolt. I don't believe there are any absolutes or here. I am pretty sure, however, that the 4-bolts were made during the first couple of years of 400 production. You hear all kinds of rumors and b.s. about how you can identify this or that part based on some external clues, but you don't really know until you start taking it apart! I've seen too many things that don't follow the rules! ALL 400 s.b.'s, as they came from the factory, were externally balanced. Like the one guy said, you can buy a bolt on balance plate to use any automatic flexplate. I will say however that the externally balanced flexplates and dampners are readily available and not much different in cost. Just go to Summitracing.com or the equivalent. I know Speedway motors sells the balance plate but I'll bet you can get that from Summit as well. For any street motor, two bolt is fine. Put studs in the bottom end if it will help you sleep at night. Keith Black has affordable pistons that will allow the 350 rod in the 400. The 350 rod has more meat on the big end; just what you want for hot rod use. You can use any small block head, just be sure to add the "Steam Holes". Use a head gasket as a template and drill away! Note that the 400 has it's own head gasket for the larger bore and steam holes. I'm running one of the aforementioned 400's now in my drive to work car. The short block was left over from dirt track racing. With overdrive, 2.73 gears and a mild cam I'm getting 23 mpg on the highway!
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