Recommended carb for 440 Duster with 4 speed?

750s Holley's are usually the easiest to find at a reasonable cost if you don't mind rebuilding.
3310 vac. Sec. Or 4778 dbl pump. Depending on your preference.
See them at every swap or in everyones spare parts stash regardless of brand loyalty.
Just beware of drilled air bleeds or missing parts that can turn them into a parts carb only.
A 600 is a little small unless it is to be used on a low rpm granny geared truck application.