You are missing one component to the original system. The locking base "plate" that is pop riveted to the body. That's what those two 1/8" holes in the body are for.
I'm not sure but these may be unique to Panteras?
At some point ( I don't know when) a plastic plug was introduced to close the bottom of the center large hole. I've asked around but most have no clue on it. They aren't on pre-L cars and most L's.
I think that the plastic plug was another of the '74 model changes/additions.
If you are intent on installing the original hardware, King here has the parts new or used that you need. He is also know as barbaram442 on ebay.
For the sake of discussion, there are other ways of attaching the tub.
True that many are just riding around with the tub just set into place. The tub has the well deserved reputation of warping to the point that the original mounting points will no longer line up if run without the fasteners.
That point right there is more likely the reason so many owners are running with the thing unfastened. The reality is that it doesn't really need to be, maybe just dowels sitting in the holes would be sufficient, but that's not what you asked.
Inquire with King for original parts, new or used.
If you are looking for other solutions, I can show you what I did, but it's not stock.