This is aimed at the first post!
I know for a fact that my knapsack straps that go across my chest had to be cut to fit so there's no excess strap hanging at the ends of them. I'm a skinny sort of fellow so everything I get I might have to adjust it to fit me. Same goes for the canteen cloth strap, I had to cut that to fit. It all depends on how your body is and what comfortable state you can achieve.
My suggestion would be to put on your shirt, trousers w/suspenders (if any) and sack or frock coat. Put on the knapsack (with everything you might want to carry) and try not to get the straps that go across your chest too tight, just tight enough that it rides high on your back. Do this and walk around the house a little bit to get the feel of it.
Have 12 ancestors that fought for the Union
"To charge the enemy or enter a battle when one knows that there is no hope of success, requires courage of a much higher order than when the soldier is sustained by the enthusiasm born of hope."
-- Colonel St. Clair A. Mulholland