72 K20 Cheyenne $10.9K
I'd look closely at VIN, SPID, etc - "replaced 400 engine with a 454" is a bit of a giveaway that it could have been born a C20 -or- perhaps it was just a 350 that was replaced with a 454 - but most sellers probably know at least the difference between a big block and a small block. Might be worth a look though - if an original K20, it won't last long.
EDIT: correction a few posts below, it's a C20 and is advertised that way - thanks Bones!