Shoulder bolts locate clutch assembly on the flywheel better than bolts that are threaded all the way to the head. The shoulder needs to extend past the mounting surface of the clutch cover apx. 1/8 in or so and the threaded hole in the flywheel needs to be counter bored a little to allow room for the shoulder.
The clutch assembly needs to be balanced with the flywheel and marked so when removed, the clutch can be re-installed in the same orientation as it was when balanced. This may be a very small point but everything will be in better balance if there is no "slop" in the mounting bolts. Regular grade 5 or grade 8 bolts will not fit the holes in the clutch cover as tight as a shoulder bolt.
A grade eight bolt will hold the clutch on the flywheel and there may not be a problem as such but a shoulder bolt will help keep things in better balance. With a slow speed Model A engine this is not as much as a problem but it can with more modern engines that rev. higher. Not using shoulder bolts can create a vibration that you can definitely feel.
Actually, these are not really shoulder bolts. A true shoulder bolt has a length of the shank of the bolt that is a much larger diameter than the threaded part. I.E. the shoulder may be 1/2 in. dia. where the threaded part is 3/8 dia. I call bolts for a clutch 'clutch bolts" where the un-threaded part under the head is "full diameter" then the threads start. I.E. the "shoulder on a 3/8 in. clutch bolt would be full 3/8 dia. (.375 in.) and then the threads start. Regular grade 5 and grade 8 bolts will have the un-threaded portion under the head slightly smaller than the nominal bolt size.
I am very fortunate to have a good speed shop just down the street that has a good selection of this type of hardware. They also do my balancing.
My opinion,
Chris W.