This project is actually quite a bit more complicated than you might realise
The very same topic has been discussed at some length on another forum http://www.navitron.org.uk/forum/
If you repeat your request there you will be guided to the relevant article - I did try finding it but "failed" However I shall give it another go and if successful, will advise
Basically your problem is that, because you are using a single voltage measuring device you will need to multiplex (switch) it across each of your cells in turn.This switching involves both positive and negative polarity to ensure you are looking at only one cell at a time. This implies that the meter (arduino ?) ground connection is NOT connected to your battery common ground but does in fact connect to the negative pole of each cell in turn. However you also need to build in some form of protection (resistors, fuses etc) to cover the eventuality that your multiplexer locks on to two cells at the same time, since this will inevitably place a short across one of the cells.
To measure current, I'd avoid using a shunt since you will want to measure both charge and discharge current. The ideal device is a hall sensor with 0amps set to give an output of 2.5 voltsDC. Maximum discharge gives, say 4 volts DC and maximum charge gives a voltage somewhat less than 2.5 volts. With 1200AH batteries I'd imaging you might consider pulling out say 100+ amps, whereas your charge facility might only be good for say around 50amps, or even less.
Best of luck with the project
Note that current through a bank of cells is the same through all cells, irrespective of individual state of charge so you can get by on a single current sensor.