youve got a 9 inch if the bolts are pointing toward the front, a 351m is a big block even though it doesnt look it, the 351 windsor and 351 cleveland are small blocks, and if you have 8 valve cover bolts its a big block (351m or bigger) if it has 6 valve cover bolts its a small block (351w or 351c or smaller), how you tell if its a 351m or 400 i dunno, i thought that 351m meant 351 modified to a 400 because every sticker on the valve cover ive seen for a 351m says 351m/400