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I have a 78 150 that I was told has a 400 in it, but the emissions sticker on the valve cover says 351m. So I was wondering what are the ways to tell the difference
There are alot of threads on this topic here. The easiest way to find out, is to measure the stroke. 4" stroke, then it's a 400. If it's a 3.5" stroke, then it's a 351. Have you pulled any of the casting numbers off of the block? Also, this site may help you as well:
