Newbie here starting my first A body

Welcome to FABO.

All parts for a 318 are completely interchangeable with a 273, except for pistons and rings. If it fits a 318, it will fit a 273. However, the 273 cannot handle larger valves without notching the bores for clearance.

You can also swap in a 340 or 360 and the passenger motor mount is the same, but the driver's side is different. I would keep the original engine on the side to stay with the car.

The Edelbrock LD4B is the best intake for the 273/318 for the street. It is no longer in production, but you can find one here and there....