This may a be a stupid question by I need to make sure

I'd be more concerned with the proof load of the aluminum threads in the manifold than the tensile strength of four steel 5/16" bolts or studs.
IMO, Use it but be safe. Proper thread engagement and condition of the bolt holes would be my main concern. Bottoming out a bolt or stud could cause galling and possibly damage the threads upon removal. And be careful to not over torque and stretch the threads.
Not that this is the case, but I don't think I'd try it in something that has helicoils installed.