small block 318 headers in an A body
The issue here is the shorty headers are a universal header that will require a good pipe bender to fit the exhaust pipe in. IMO, skip that header.
The common headers are a major pain to install and really require a freind, and the car to be lifted very very high up and a crane to tilt the engine at the same time to install. I hate the common header but, they are cheap, and easier to resell.
Since your going to do a BB swap later, do not invest to much coin into the teen.
Also, since your doing a head/intake swap, after the heads and intake come off, try to do the header then. It'll be a good bit easier to do the header when a lot of weight and iron mass is missing.
The common header can be purchased from Summit racing and is close to being the cheapest header around.
The header style is a one size fits all makes thing.