400
A 400 in³ small-block was introduced in 1970 and produced for 10 years. It had a 4.125 inch bore and a 3.75 inch stroke. Initial output was 240 hp. While popular with circle-track racers, the engine was prone to cooling troubles if cylinder heads from other blocks were fitted without proper modifications. The 400 produced a lot of torque at low RPMs compared to the other displacements and was used primarily in pickup trucks and the large B-cars (Caprice/Impala) but also found use in Vans which could not take a big-block motor. The 400 was eventually discontinued due to its inability to meet EPA emission regulations in place in the early 1980'*.

The 400 was unique compared to all other displacements for 3 reasons:

Due to the siamesing of the cylinder bores in the casting - there was no space between adjacent cylinder liners - this block was rather rigid and not prone to the cylinder bore flexing of the other displacements.
This block used a much larger 2.65 inch main bearing assuring that crankshafts from all other blocks would not fit without special oversize bearings.
This is the only small-block that used a 5.56 inch long connecting rod compared to the 5.7 inch connecting rod in all other displacements.
While Chevrolet never produced a factory performance version of the 400, it is highly sought out among racers who participate in the very popular 406 racing class - so named for the displacement created by a minor overbore. Destroking a 400 block with a 350 crankshaft creates a 377, a popular combination for dirt track racers. 400 blocks are nearly impossible to find as salvage today and as such the racing market is fed by 3rd party manufacturers who offer improved versions of the block.