The bolt heads break off these engines and fall off without even trying to remove the bolts. Thus the broken studs being there. GM used crappy hardware. I definitely used ARP when replacing mine. The clamps work fine if you don’t want to pay someone to remove the broken studs or can’t get them out yourself.I would drill out and retap to install new studs.
Next time, instead of risking a broken stud, do this. Using a small wire brush and pick tool to remove any rust from the nut/bolt joint. Then apply some Aerokroil and let it soak in overnight if possible but at least for a few hours. Then using a wrench or socket alternately try loosening and tightening and you’ll start to get some movement in the treads. Spray more Aerokroil then you should have a good chance of removing the nut or the entire stud.