Early A exhaust manifold questions

First, I’m not clear on what the goal is.

As far as 273 exhaust manifolds the stock 64-66 are fine to use on a stock power type of 273-318.
If you want to make a lot of power there might be a few other model year pieces that work but I can’t really give any advice.

Sounds like you have a ‘65 Valiant and are thinking of a V8 swap. Unless you already have a complete donor for ALL of the parts needed I wouldn’t bother. It’s a great idea but why not just have a nice running slant? There’s plenty to do with a ‘65 ( because it’s 50 years old) to make it a nice driver like front disc brakes, don’t forget dual master, wiring upgrades, suspension refresh... etc.
Just think about your goal and make it manageable.