It's not the max amp-hour but what you needs and vehicle requires and what will fit. The standard 72A/h battery is ok, it's the same for the batteries CCA.
 
Not sure on the V8s but on my TDi ive got a later Rangerover battery fitted I think its around 100 ah with cca of around 1000.
I had to take the side rad bracket off for it to go in and then fasten it back on after the battery was in place.

Measure the room you have and fit the biggest you can.
 

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