Welcome. You’re on the right track swapping what looks worn. Hard to determine the clunk you described without hearing it. I would swap out fronts with bilstein 4600 or 5100’s. Basically the same but the 5100 has some height adjustment. Certainly can get a clunk if blown. As far as caliper mounting bolts go, those were installed with factory loctite. My 1/2 inch aircat pneumatic with over 1000 ft lbs of torque struggled with those. A big breaker bar will index them off. They will sound as though they are snapping as you rotate them off. Nissan wasn’t playing when they put those on.