The 640 is a SAE rating, the better rating is EN as it is much more realistic as in the test criteria is cold performance cranking etc, and a 640 SAE would be roughly 225A on EN rating.
Even if the 640 started the jcb I bet it would bend the plates!!
On my 2.1 pinto sierra, I cut the battery tray end off and had a 120AH, 950A mercedes G wagon heavy duty in there, it nearly spun the car when starting!!
tabetha