Bolts or studs and why

I don't know about the torque specs, but the two end holes in the heads go through to water. You need to use the factory studs there. You cannot use hardware studs, they are different. The factory holes have a chamfer around the hole. The factory studs have a bevel that acts as a seal against the chamfer when they are tightened. I do not know if aftermarket heads have that chamfer or not, but if they do the correct studs should be used. This is why people have trouble getting bolts to seal even with sealant.