are those water slides? if so you can install them anyway. just apply some water slide adhesive and softener. done this on car models putting stickers from underside (like it suppose to irl). softener helps shrink the decal for better grip but use it sparringly.
Wow 1st time i see someone putting a head from fixed figure onto an action body.
I have seen folks cut heads off statues onto 1/6 body but never though of simply plucking 1 removable head up onto a body. You're a genius.