If you're looking to get her running, get out a do a few trips and then sell her; the $2,400.00 engine will do. If you want her to last a few more miles down the road and enjoy her without having to spend the big bucks getting the engine out again, an engine from a reputable builder maybe the way to go.
There are a few builders that would relish putting something so classic like an Airstream on their website and would build you an engine to suit your application. Four bolt main versus two, the correct camshaft for good torque versus high end horsepower. Roller rockers for endurance, fuel injection, etc, etc.
Dropping in a block designed for even truck use may not be enough for a rig weighing 12,000 lbs and pulling a toad.
Having someone design an engine for your Airstream may save you money in the long haul with better economy, more power and longevity.