I have only ever seen one pair of saddles with the bolt heads welded like that. Funny thing is, it was one of my very first restorations and I re-bushed the spring saddles and had a friend 'put back the welds', lol.
Personally, I would not replicate it at this point, unless I saw a pattern of other cars built around the same time, same plant.
On the pace car I restored, the bolts were different than the later style, but I'm pretty sure they were splined also. They were not welded either.