I think the distinction needs to be drawn between weight lifting for strength and bodybuilding. To be honest I've never sparred with a bodybuilder who could hit very hard. Also there's two ways of generating power - internal and external. Lots of xingyi nowadays claims to be internal but if you absorb a hit from one of those dudes they are just using the external or muscle power. Internal power comes from your weight coupled with tendon strength which is the result of lots of static stance training in the santi posture. The difference between internal and external strength is that the external puncher will throw 6-8 punches in succession and then the power will begin to decline. The internal puncher can throw a punch that will knock you off your feet and he can throw another 50 or so in succession with equal power because he's not using his muscles or not beyond any more capacity than it takes to support the arms in the effort. Punching power is a balance between physical force through the muscles (minimal) and "soft" rigidity of posture. A "perfect punch would be a balance between the two variables which is optimal. Weight training is one way to go about it but in my experience an equally successful method is good ol calisthenics, push ups, pull ups. I tend to shun weight lifting in my own training because after I lift weights I need to stretch again to avoid tensing up. I usually begin my workout running in place for about 5 minutes then I do 100 jumping jacks to get the cardio working. Then I work my abs and midsection with some modified sit ups and leg lifts and I finish up with push ups, squats, and qigong which targets the arms and shoulders. The stretching is built into the workout itself so I don't have to start backtracking and doing things twice which I hate. One of the things that I have found to be of immense untility to overall strength is static stance training. I stand in both a left and right santi posture for 5 mins each side every day. There are no immediate power gains but after 6 months or so of this you will notice an incredible overall increase in strength. Lots of guys stand 5 minutes in the posture and think "Oh man I could do 10 or 15 minutes next time." But then that quick of an increase comes with pain so they get discouraged and just quit altogether. Moderation in any of your training will take you much ****her than balls to the wall.
Also if your in Kung Fu eventually you've gotta talk about qi. I am no expert but I do know how to recognize the effects and work with it. I tend to view qi as more of a guide or a rule as to whether I am doing the techniques correctly with the right posture and tension so forth.