Generally speaking the joint will not be as strong as the original piece of wood. I am not familiar with the LT 40 but if you can put the joint at the outboard end of the spar it probably won't be significant. Also you can put doublers on each side of the spar joint. A piece of 1/8th balsa about 1/2 inch long ought to work. End joints on spars are usually done with a "scarf" joint. It is a diagonal cut with the length about 5 times the height.