You seem to be a little confused, mixing descriptions between different components.
Small block: Chevrolet engine manufactured from 1955 through today in many different cubic inch sizes in 3 different generations of engine types, including but not limited to these from Gen I....265, 283, 302, 307, 327, 350, 400.
Big block: Chevrolet engines manufactured from 1965 through today in many different cubic inch sizes. This motor is physically larger than a small block. Engine sizes for the first series of production (Mark IV) include 366, 396, 402, 427 and 454.
Short block: Engine block, crankshaft, rods and pistons all assembled and ready for cylinder heads to be bolted on.
Long block: Engine block, crankshaft, rods, pistons and heads all assembled and ready for intake manifold to be bolted on.

The 350 will slide right into place on the same motor mounts that kept the 327 in place.