I made a project like that, but never got it to totally work.
The ground strap is not a good choice, because the resistance varies a lot with temperature. Temp varies a lot with current as well as engine compartment temp. You would need to modify it with 4 points too or all you'll be reading is unpredictable contact drop. Also, the voltage really is just too low to measure accurately.
A shunt is a good choice. Some of the surplus catalogs carry a 200 amp shunt. Since modern alternators can put out well over 100 amps, this is a good idea. The Hall effect devices have a bit of trouble if you exceed the rated current, it can permanently magnetize them (causing an offset error), and their offset error is usually large anyways. So using a 100 amp Hall device on a system which might be able to exceed 100 amps (even briefly) is a no-no.
I used a 24 bit Linear ADC with no preamp. This is handy since they can measure 0.3 v above Vdd, so if you hook Vdd to 12v, then use a -5v reg to get 7v and use that for the ADC ground you can measure directly. The ADC output then has to be optically coupled to ground reference. If you used an op amp, you'd probably need to generate a positive voltage above 12v since op amps can't read voltages above (or usually near) Vdd. Note- it would also have to be an extremely low offset op amp, a chopper like LTC1013.
The Linear ADC was disappointing, I could not make it live up to its specified offset and offset thermal drift spec. Also the noise was pretty high, and it's not fast enough to take a crapload of readings for averaging.