71 Valiant source for upholstery?

MCPObr44 said
Do you suppose a Coronet has similar sized seats to a Valiant?

Unlikely.
https://www.lov2xlr8.no/brochures/dodge/do71/bilder/19.jpg
The width of a Coronet is almost 10” more than a Dart.

Do you have Coronet seats to swap over?