I don’t have a +8, but can also recommend the Odyssey range of batteries.
My 4/4 has a PC925 mounted under the bonnet, against the bulkhead.
My last one, still had life when I changed it after 8 years.
Well worth investigating their range.
The PC 925 is a bit larger than the one I use PC-680. But I suspect it will fit better and give more cranking power. My +8 Bitsa is not a Morgan that was assembled by MMC and I did not want a battery behind the seat, thus a smaller battery, and cheaper, on the bulkhead. I also have very limited need for a large battery. No AC, Radio, Driving Lights etc.