On the bypass, the main factor is where the throttle "wants" to be at while the engine is idling. If it is open too far to get it to idle (causing the dreaded "nozzle drip") then the bypass holes are added. Obviously a stock engine with the original stock carb would not need bypass but any thing other than this MIGHT warrant the enlarging OR adding of the holes. Let's say that you can get the engine to idle but the throttle plates are open too far, causing a bit of nozzle drip or rendering the idle screws to the point that they have little effect. At this point you could pull a small vacuum line to determine if the idle bypass would be needed. If the engine idles much better and is tunable with the screws/throttle position, while a small vacuum line is disconnected then you know that you need the bypass. If after this is accomplished, it seems that the screws still have little to no effect, then the idle screw hole then should be enlarged also, however, enlarging the idle screw holes slightly has not too many bad effects either way, it just effects the amount of the turning of the screw itself. Larger=less needs turn the screws. A good example would be going from a 305 chevy and putting that same stock carb on a 454 bigblock. I would guess that in this example you would need to add bypass and enlarge the screw holes.
