idk bolt sizes i thought they were just held on with plastic tabs
zipties will hold your bumper on no prob theyve held my lip on for over a year and a half now and my lip rubs everytime i come out the driveway
if your really worried you can rivet it on
my buddy riveted his bumper on his 05 impala and he has two alpine xs in his trunk and needless to say it hasnt came off yet
or you can just play trial and error til you find somethin that works out
Just use self tapping screws you can pic up just about anywere, screw the bumper in place. You should be able to stand on the frount bumper without it falling off, and then you know its on good enugh.
