Correct hardware for 65 GTO Exhaust Manifold Flanges
 

Service manual shows bolts being used in the drawing (3/8-16 x 1.375" LHS & 3/8-16 x 1" RHS) for attaching headpipe to exhaust manifold

My car has studs and at least one repro house shows studs as the proper hardware

Which is correct? If bolts, which Grade? 5? and where is best place to get them? Specialty nut/bolt supplier or local hardware store

Thanks