better triple check that displacement. in 1967, the Le Sabre was the small block big car model and the "Le Sabre 400" came with a 340ci small block.
the fast way to tell the difference ( because Buick small block 340s and 350s and big block engines have almost the same deck height ) is by counting valve cover bolts. 4 bolt valve covers indicate an early SBB ( 215, 300, 340 ). 5 bolt valve covers, 3 bolts on the top rail and 2 bolts on the bottom mean a BBB ( 400, 430, 455 ). 6 bolt valve covers are particular to a 350 in the Buick engine families.
even if you've got a 340ci, a 455ci swap is still pretty straightforward. you need new engine mounts and a big block fan shroud. everything else should match pretty close.