Hi Graeme and before you move anything, do some measuring.
To my knowledge, the only way you will fit that size of battery, is the modify the battery compartment wall.
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By my measurements only the ribs along the front bottom edge will have to be attacked. Its a Remco RM 12-80 L=259 W=165 H=208 with bolt holes and no posts for the connections.
With those measurements, you should be fine with a little massaging of the fins!
Cheers,
Gordon (enjoying a red or three in McLaren Vale!)