You need to get out hard to develop a rythym and good pace for the rest of the race. The 400 is a sprint, but you shouldn’t really go balls out the first 100 either. That first 60m should be a hard acceleration, then maintain from there til you reach the second turn; once you enter into the turn is the crucial part of the race (IMO)… If you can run a fast 300, you will be in good shape. Go for the 300! The last 100 will pretty much take care of itself if your training has been good.
There are a lot of strategies out there, but I think it all depends on what you feel works best. It should be rehearsed in practice so you don’t need to be making any adjustments before or during the race. You should be able to work the second turn aggressivley coming into the final stretch to bring you home.
Your question about your physique is very general. Are you asking should you lift weights? Should you put on size? Might be better to ask that sort of question in the Ask Us section or the Fundamentals–> Strength section of the forum. Hope that helped!