Good question and I wondered that myself last night.
Looking up rear discs on Rockauto they do seem to be vented. It does give me a thickness of 26mn so I can check that with what I have, perhaps wrong ones are in there.
Done a few emergency stops on the drive way. Checked for any brake leaks. We are good to go.
A little drive around the block at low speed. Was some noises like pads rubbing on the disks. That may just clear with driving. There was some smoke under the bonnet, like oil seeping out of the rocker cover(not sure).
Darren recommnended some Stop leak to revice old gaskets. https://rislone.com/international/wales/
wouldn't be surprised if rocker cover gaskets need to be replaced as been siting for so long . the link above for the seal restore wont do rocker cover gaskets but will help will valve stem steals other brands of that stuff are available,
do you know if rocker cover gaskets are rubber cork or silicone type???