Eddym

I am 1 month into my first build from plans. The aircraft is a 1/3 Howard Pete designed by David Anderson. I am cutting all the parts my self. I have almost completed the bottom half of the fuselage. The gear is complete and I have just fitted the wings. My question concerns balsa grading. The plans call out 4 to 5 lb. balsa. I know it is a little late in the game to be asking about grading but the more I look at the 1/8x3/8 fuselage stringers the more I am getting concerned. this will be my first time using coverall and am concerned about breaking stringers during the finishing process. The balsa I purchased was not graded. I know the difference between soft:no grain you can crush it easily with your fingers. hard: has a grain and not easily crushed. I wish I knew more in order to make better selections. Can someone explain the grading system (either standard or your own). Also we're is the best source for balsa?
Thanks