Why was it necessary to call the 400 ci engine small block, when there was no big block 400?

Because there was a big block 400. It was actually a 402 but some fender badges in early seventies simply said "400". Could have been a small or big block.

Done wrenching is right. In 1970 you could get a small block 400 or a big block 400 in a Monte Carlo. Yes, it was a 402 but Chevy labeled it a 400. I had a small block 400 70 Monte Carlo and I had to explain the difference many times when I bought parts for it at the chain parts stores. It was confusing back then and it still is now.

I guess just to eliminate confusion, since there WAS a 402" big block.

"I've got a 400" block"

"OH, you mean a 402"?

"NO, a 400"

When I used to run a 302" (chevy)small block, guys were always telling me "there aint no such thing as a Chevy 302. Only ford made a 302. Maybe you mean you have a 301" (a bored 283)

Because of the fact of the whole 396 big block mix up and change around in 1970

The 396 changed in 1970, GM bored the block .030" larger and now it was a 402 big block. But during that time chevy used 396, 400 and 402 lables, marketing and badges for that same engine

Then you also have gearheads that calls an engine buy its true size. Like 355, 360, 406, etc depending on bore size. So you have a mix up there with the 400 SB and 400 BB. Bored out 396 comes out to 398 to 408 depending on overbore

The Chevrolet small-block engine is a series of automobile V8 engines built by the Chevrolet Division of General Motors using the same basic small (for a V8) engine block. Retroactively referred to as the "Generation I" small-block, it is distinct from subsequent "Generation II" LT and "Generation III" LS engines.

Production of the original small-block began in 1955 with a displacement of 265 cu in (4.3 L), growing incrementally over time until reaching 400 cu in (6.6 L) in 1970. Several intermediate displacements appeared over the years, such as the 283 cu in (4.6 L) that was available with mechanical fuel injection, the 327 cu in (5.4 L) , as well as the numerous 350 cu in (5.7 L) versions. Introduced as a performance engine in 1967, the 350 went on to be employed in both high- and low-output variants across the entire Chevrolet product line.

Although all four of Chevrolet's siblings of the period (Buick, Cadillac, Oldsmobile, and Pontiac) designed their own V8s, it was the Chevrolet 350 cu in (5.7 L) small-block that became the GM corporate standard. Over the years, every American General Motors division except Saturn used it and its descendants in their vehicles.

For the same reason they called a 350 a small block. Because it's a small block....