Can't really comment on the originality issue (I'm sure the guys in the Humber Rootes Section will know however) but the engineering fundamentals may be helpful in your decision. The physical size of a lead acid battery is determined by two things - cranking amperage is controlled by the internal resistance of the battery and the surface area of the plates defines this. So the higher the CCA rating the bigger the cross section of the battery. (Hight x Width). The ampere hour (ie storage capacity) of the battery is a function of plate thickness and number of parallel plates in each cell. So the AH rating of the battery generally will influence the length of the battery.
Now voltage at the starter motor is the critical thing in terms of a battery located in the boot. This "voltage drop" may well be determining the size of the battery cables rather than current rating. Now this engine probably has a relatively modest starting current (low CCA) , so someone may have fitted a large battery (high CCA) to get rid of some battery internal resistance and help the voltage at the starter motor if the whole thing is a bit marginal. Make sure all of your terminations are clean and tight (battery and any crimp connections in the wiring) , the solenoid contacts not burnt and the starter brushes clean. If all of this is good then you MAY be able to use a smaller CCA rated battery more in line with the age of the car. Putting the battery in the engine bay obviously dramatically shortens the battery cable length so gets rid of voltage drop to the starter.
So why do people but batteries in the boot ? Well space (sometimes a problem in modern cars but probably not yours) , weight balance (not really a consideration here) but the big one is to this day lead acid batteries hate extremes of heat, cold and vibration. Cold they don't deliver their CCA and hot simply cooks them (remember, a battery is a chemical reaction) and vibration simply shakes the plates loose. So in terms of a long life getting them away from a hot engine bay and engine vibration is not a bad idea. Pros and cons either way. Hope this helps.
