what is needed -convert 65 Dart 273 to 340 or 360

In an early A the only high flow manifolds that will work well with a bigger motor is the 1960s 340 driver and passenger's manifolds BUT ONLY IF you have manual steering. The 60s 340 driver paired with a 360 magnum passenger manifold will work even better because it gives you easier clearance and access to the down pipe and it's not nearly as expensive.

The 340 manifolds have a 2 1/4" exhaust hole exit while the 273/318 exhaust manifolds only have a
1 5/8" exit hole (BIG DIFFERENCE). The 360 magnum can be machined out to 2 1/4" to match the 340 driver. The 70s 340 driver would probably have clearance problems with steering box and steering shaft.

Treblig