ahhhhh....the Age Old Question.
It is BASIC and SIMPLE if you think about it.
273-2bbl, and 318-2bbl.....have the smallest valves 1.78/1.50, lightest duty springs (.450"), smallest ports.
68-70 (X,O,U castings) 340's have 2.02/1.60 valves.... .500" springs, no rotators, and monster...