Hi,
Do you have an inspection shop that you use currently? I ask because I'm sure you could ask them specifically what is legal and what isn't . Also some shops,Like one I use(I'm not naming names.lol) Don't go over the older stuff with a fine toothed comb,meaning I could just about get a car with opened headers passed . Alot of times it boils down to how lax a shop is.When I did inspections we were pretty forgiving in terms of exhaust systems.
For 1965 you might be pretty safe,look at the stock exhaust on a 1965 Shelby Mustang ,It was straight pipes into glasspacks and dumped out the sides in front of the back tires.
I'd say ask the DMV, But they'd probably make you put cats and a single exhaust on it.