Grey plastic were also used on the G-Body (Regal/T-Type/Grand National). Points have been known to be deducted if the caps were not grey while being judged, right Roberta? ;-). They are available from several G-Body parts suppliers.
No, that is definitely not a 1971-73 Riv frame/suspension. Parts may be, but it seems to be a combination of 1956 and parts of the Riv frame/suspension.
Another thing noted in passing...at least sweep out the trunk before you take photos.