thats a 1/8" rivet, correct?
then you take the one thats too long and cut it down to 3/16" of rivet sticking through the hole! your other rivet may be too short.
rule of thumb, 1.5 times the diameter of the rivet is how much of the "tail" of the rivet that sticks through. i am running into that alot on my 7 kit.
do you belong to an eaa chapter? if so see if they have a Tech Counselor, he can answer your questions and assist you throughout your build. if your eaa chapter doesn't have one, join one that does, its your life!