If the splined area fits the hub, the length of the spines section shouldn't keep the drum from sitting flush with the hub. If the hub has been drilled at some point and oversize studs were installed, that could cause that problem. Either way, I'd enlarge the drum hole just enough to let the drum pass over the studs and re-swedge. Then put the assembly on a brake lathe (with good bearing races) to make sure everything is square and not out of round. Before you get too involved, I'd verify that the outer bearing races fit snug in the hubs. That's a very common area for problems on 9" brakes.
