RE: BH model Piper J-3 Cub, crashed...
often the thrust angles are built into the firewall. if they are already in the firewall, you don't need to shim any more. i think the main issue is to the get the CG right. if it still flies funny then you can work on the thrust angles. i'm sure the directions will tell you if the thrust angles are built in.
good luck!
ps 5 ounces is nothing on this plane!