To be honest, the alternator does look a bit too small for the engine? Couldn't find anything on it to tell me what is so wonder if would be worth getting a higher output alternator.When engine is running you should be nearer 13.8 -14.4v
Engine off battery should be min 12v but ought to be around 12.5v
If going for the Optima then you need a red top for starting engines. 550cca ought to be enough for starting, but the higher the number the better obviously.
I'd be looking at your alternator output - it sounds suspect.
When the engine revs increase the voltage does go up higher than 14V.
But, if memory serves me right, I will need to check it, on idle it shows over 12V.
Engine off, battery reads 12.5V
Ill get it checked tomorrow if I have time and report back here.