1976 Duster Rear Bumper Inner Core & Front Bracket Question

I bought a bumper for the rear of the '76 Duster, but no mounting core........ The old core must have gone to the boneyard. Any suggestions? And, I'm ready to mount the front bumper, but the previous Bozo hacked off the rear of the left-front bracket so I have only ONE bolt hole for the bracket instead of 2. Do you'se guys think one bolt on the drivers side will hold that bumper in position under normal driving conditions...... ?