Ok so the Optimas are close to double the price over the AC Delcos. Not a fan of Century batteries lifespan with the ones I've had, so Delco it is!
This is the battery box I have, their website says;
Projecta Battery Box - H: 200mm - W: 190mm - L: 330mm
My current battery is - H: 225mm - W: 172mm - L: 305mm
and that bigger AC Delco I wouldnt mind is - H: 234mm - W: 172mm - L: 330mm
from the other pics you can see the wires and breakers at the top of the box, so unsure if that 10mm is the breaking point of the terminals touching it.
I sent Projecta an email asking what the maximum dimension it can fit internally and they just replied the above measurement which I know it fits bigger batteries so they were zero help.