Yes I think you’re right. 215 of some sort. Rover bought the rights to the design from GM for the Range Rover.
I agree, it’s the Olds variant. The giveaway is the top of the head is relatively parallel to the deck, the Buick 215 heads used a casting with a different angle at the top. The Pontiac Tempest used the Olds 215.
If you have a chance to grab that one-year-only ('63) alternator bracket, you should. No idea what that extension is protruding from the crank pulley, though.
A buddy has 3-5 of them.a couple are the high compression variants, even has a blower set up for one!
If you want to dive a bit deeper, @In_The_Pink is building a great car and has a bunch of good pictures of the engines and parts. https://www.jalopyjournal.com/forum/threads/mish-mash-nash-52-rambler-wagon.1291532/