"Heart murmur" is a fairly ambiguous term, usually for a mild flaw in the valves of the heart. You'll want to ask your vet about the severity of the murmur and what the long term prognosis is, what kind of exercise is appropriate, how strenuous, etc. I don't know that I would expect very good nutritional advice from a vet because most of them only push Eukanuba or Science Diet, neither of which are feeds I would recommend for a healthy dog, let alone a dog with a health problem.
Check out the nutritional information at
www.greatdanelady.com. It should be a great help to you.
Good luck with your puppy. Oftentimes the ones with a problem can be so special and bring so much love into your life; they need your care and love and give so much back to you. Let us know what the vet says, please.