What carb is this? What are my options?

That's a Carter BBD. Looks to be off a 73 or later 318. You's have to check the number to be sure.

As for options, bear in mind that early 273's have a different intake bolt angle than later 273/318's. Manifolds for the earlier engine are pretty much limited to the factory 4bbl (not so great) or an Edelbrock D4B (good, but getting hard to find). The later engines could use any commonly available small Mopar intake.

As for a carb, if it was a stock 273, I'd use an Edelbrock 500. If it was beefed a bit, the Edelbrock 625 would be a good choice.