I recently bought a complete 4.0L v8 from a 1999 Land Rover Discovery, and these are the stock headers I removed from it:
7356C409-ED91-450F-B11D-F1C5B6584104.jpeg
Note they aren't mirror images of each other. The header on the right side is 4-into-2, with the 2-into-1 joint somewhere past the joint on the down pipe.
I'm thinking if you can find two Discos of that vintage, you could have a matched set.
I'm not sure if they would clear the outside edge of the MGB frame rails in an RV8 through-the-wings exhaust configuration, but I bet they would with a slightly wider Buick 300.