I used a Braille B2015 in my 65 4/4. This is a lightweight (15 lb.) AGM battery similar to the Optima.
Braille offers a very nice aluminum bracket that can be used to hold the battery down flat on its side. Clean, simple design that, to my eye at least, looks "right" in a Morgan.
Had it about 3 years, so no data on longevity, but it has no trouble spinning a 1.6 motor with 9.7-1 compression and a high torque starter. My electrical loads are light. Running and signal lights converted to LED. No radio, radiator fan, or heater. If you have these things a slightly larger battery may be called for.
Battery and bracket are available from a number of on-line sources in the US. No trouble shipping an AGM battery.
Hope this helps,