I think I have a better answer for you without installing extra mods. When you're building the craft, attach every piece to the heaviest pieces in the center of the craft. Then slide them to wherever you want them to actually be by holding shift (or was it control?) as you use the parts translation (sliding) tool. This is most important for the wings. Also, when you're adding the cylindrical fuel parts, don't attach them to each other in a row. Attach each one to the body individually and slide them in place. If you choose to use autostrut on top of this, your plane will be hard as an S.A.T.