broken bumper bolt replacement 280zx

I'm putting the bumpers off of my '82 280zx parts car on my '79. When I took the rear bumper off, the four bolts were pretty rusted up, so I took it off with the shock brackets attached, figuring I could clean the bolts up with a dremel wire wheel. I cleaned them, but it didn't help much, and I sprayed them several times with pb blaster, but I still broke off four of the 8 bolts. I thought about drilling a hole behind each bolt to get access to them, but I figured I should ask the experts first. Is there and good way to replace these bolts? The rest of the bumper is in very good shape, so I hate to scrap it. My '79 ones are shot, but the shock mounts are real nice. The mounts on the '82 are bent and rusted, or else I would have used them.