New position on the Shadows
"The main change to the engine was moving the position of the sparking plugs to above the exhaust manifold, which made them easier to remove at a service."
Source: RREC website

Recomended type (Users manual)
Champion N.14.Y That is 14mm non detachable plugs.
Source: Owners Manual 1968 - 2nd ed - 1975 reprint

Cleaning and changing
Cleaning every 6.000 miles (10.000 km) ; changing every 12.000 miles (20.000 km).
Source: Owners Manual 1968 - 2nd ed - 1975 reprint

Actual type
Champion N12YC
Source: The Rolls-Royce Forum - Australia

Setting the gaps
Depends on the engine size : 0.025" gap for the 6.25L engine / 0.035" of a 6.75L engine Grease the threads with graphogen grease
Source: Owners Manual 1968 - 2nd ed - 1975 reprint

Iridium alternative
NGK Iridium GR4IX Supposed to last longer
Source: The Rolls-Royce Forum - Australia