The 327/375 FI engine was always a Corvette only engine.
The highest HP 327 engine available in full sized Chevrolets in 65 was the 327/300, itself a sleeper as it was underrated.
Higher HP 327 engines produced HP at higher RPMs and were cammed to do so. Manifold vacuum was deemed unsuitable for vacuum accessories and low/mid range engine torque was unsuitable for automatic transmissions, most common in Impalas.
The 409 was sold in 1965 Chevrolets until January (February?) when the 396 became available. The 400 HP rating was the same as the HP (big head) 64 version with a single 650 CFM Carter AVS (AFB) carburetor. GM dropped the 2X4 (425 HP) option in 1964, though quite a few were retrofitted.