Actually, it doesn't matter what engine the car has as its pre-2001 and goes on capacity alone, there's only the "less than 1549cc" and "1550cc or over" bands, so whether or not it has a 2.5 or 3.0 or 3.2, tax would be the same if it was registered before March 2001.
In fact, I'm pretty sure if you got your GTA bored to 3.7 it would still make no difference to your tax.