i would drill them and use screws,epoxy can crack from vibration.also if you decide to use a bigger engine they will mess up your wood trying to remove them if you use epoxy.try to get them to the tale as far as possible i drill and use ca on the balsa after i have tapped the hole with the screws I'm gonna use.try to put the weights on the bottom of the tale as far back as you can so you wont have to use as much....happy flying