Hi Al, There was no definitive year or chassis number for the fuel tank relocation. RR served 3 distinct markets and the exhaust, emissions and more areas reflect this.
For example: A 1978 RR Shadow for N. America at chassis SRH26373 has a bulkhead location. Japan + Australia are floor mounted."Other markets" have the floor location as well.
During this era, there was alot of emission and crash concerns for every market, hence no consistency with exact year changes to parts locations.
I have dealer master parts catalogs for all service parts and the one for exhausts is the most complex since it deals with emissions and changes to the fuel tank location.