The Bonneville seats and door panels will bolt right in, but will have a different upholstery pattern than the Catalina parts. I'm not 100% sure on the rear seat side panels, but I think they will work as well.
In 1968 there were 7,339 Catalina Convertibles and 7,358 Bonneville convertibles made.
If you decide to restore your original upholstery and door panels, two potential sources of repro material are Harry Samuel at
http://www.autointeriorsbyharrysamuel.com/ and SMS Auto Fabrics at
http://www.smsautofabrics.com/
Also, check out our sponsor's catalog for full size Pontiac parts. Ames Performance also has a full size Pontiac catalog.
Welcome to the Pontiac world!