Have you tried getting your arm and hand back there? If you can reach the spark plug boot, you got it made. Twist and pull to remove spark plug boot. Use a spark plug socket and rachet, proper extensions, maybe a breakover, maybe a universal joint. Whatever it takes.
Some people rock the engine forward to get at rear spark plugs. Some makes and models, it might be easier to get at the sparkplugs from underneath. Whatever is easiest for you, thats a judgement call.