The centers of the new drums are cast iron and are thicker than the original stamped centers. Therefore the original studs are not long enough and I am not aware of any longer studs of the original style. None of that really matters because it is not a good idea to swage against the cast iron center, as it will create stress and the drum will sooner or later crack around the holes.
We instead use modern serrated press-in studs that press ONLY into the hub
flanges, leaving the drum to be a slip-fit on the hub, which also allows removal of the drum for servicing without removing the hub. The exact required studs
to do this operation are not available, so the holes in the hub must be enlarged and the holes in the new rear drums must also be enlarged to clear the stud shoulder. Also, a flat must be ground on the stud heads to clear the boss or ring on the inside of the hub, which also serves to prevent the studs from ever rotating in the hub holes should the serrations fail to hold them.
Since the introduction of new drums, there has been significant confusion as to which studs to use, and in at least one case, the studs provided would not work no way, no how. I am not sure whether that is still the case, but that situation led us to long ago procure studs that ARE workable, although they do require the above modifications. I do not believe that studs that will fit the existing holes AND be the correct length needed are available. Mac's has noted in their catalog which studs to use with the new drums. I am a Mac's dealer, but have never tried the studs that they now specify, because we have always used what we came up with originally and have been successful.
The above applies only to the hubs where the drum is mounted on the OUTSIDE of the hub flange, with the hub flange being concealed inside the drum. It DOES NOT apply to the drums that mount on the INSIDE of the hub flange, and where the hub flange is visible when the wheel is removed. That is another can of worms...