1968 barracuda interior for 1964 plymouth valiant

Doubt it. Are you talking about a notchback Barracuda? It's probably too wide. And the fastback rear seat would be too narrow and completely different, same problem as the 64 Barracuda.

I don't know what the upholstery pattern is like on a 64, but an upholstery shop could probably make up custom seat covers with the correct stripe pattern inserts.