Will adding swimming to my heavy lifting make me bigger, or will it stunt my growth?

Better: If you are 47 you are done growing. The swimming will make you better, as it will help stretch the muscles which you have constricted with the heavy lifting. Examine your rationale for the heavy lifting -- you will not keep it for life, and it's tough when the max gets less as you age. The lighter wt. And higher reps stays longer, more practical for everyday life, and better cardio results.