It's all just old tribal knowledge.... 340/440 fast. 318/383 slow. If you ever meet a true-blooded 340 guy from the 70's, they will denounce 360's like the plague.
My uncle is like that, 340 or no engine, 360/383 are boat anchors to him. A 318 is for putting leftover 340 parts on so the car is usable while working on the 340.