440 / 727 swap in a 73 duster

It fits.
As long as it's a standard 727 and not something weird like a motor home tranny, you're good to go. 727s were standard behind 340s in A bodies.
Is it RHD?
It's LHD currently, but I'm converting it using a local VE Chrysler Valiant dash and firewall with an aftermarket RHD tubular k frame.
The VE / VF/ VG Chryslers we have down under are basically just a RHD Dart Swinger which makes it really easy.