Backseat interchange

Help:
The last 340 Dart that I bought has a set of Duster bucket seats and the original Dart rear seat.

The problem I have is that I want to re-upholster both seats by buying a set of pre-manufactured covers . I'm changing the interior color.

I was going to buy a set of Duster covers but I need the size of the Duster rear seat to see if they can be modified to fit the Dart seat.This is the recommendation of my upholsterer

Can anyone help??Thanks, Dennis