440 / 727 swap in a 73 duster
A 727 came on the back end of a 340 in an A body and the floor pan was no different between a slant with a 904 or a 340 with a 727.
The bellhousing is likely different between a big block 727 and a small block 727 but the rest of it behind will be the same.
Besides, for 3 years a big block was available in an A body with a 727 so you will have no problems. Motor mounts will need to change (maybe K frame unless you’re using conversion mounts with your existing slant 6 k frame).