383 or 318

If you like scary, big block is the choice in an A-body.
A friend of my dad was a 1970 340 4spd Duster owner, in 1980 I let him drive my 67 Fastback with the 383 Torqueflite and 4.57's.
Within 6 months he bought a 68 Fastback to DROP A 440 in. He is the guy that built me a nice set of motor mounts from a diagram he found in Hot Rod.
He was an old street racer from Cleveland, Ohio in the mid 70's. He never built another small block again.
He's 82 years old now, still has the 440 68 , last time he ran it at the track was 2015 running 9.10's.