If you don't want to put the ground cable back on the a/c compressor, then I might suggest the front intake manifold bolt. Considering that the capacitor for the ignition points usually attaches to the bolt that holds the coil bracket to the intake manifold on many years of Chrysler V-8s, putting the ground cable for the whole car on the other bolt, which is electrically like attaching it to the same bolt due to the bracket that both bolts touch might not be a good idea.
The front intake manifold bolt, if the battery is on the driver's side, is where later model ground cables were attached to the block. As you mentioned, having a clean area for the cable end (if it's not there, you might add a toothed lock washer between the cable and where you mount it and then a flat washer on top of the cable end under the bolt head for good measure and so that you will not torque the cable end directly when you tighten things down) is necessary for the charging system to operate to its design capabilities. The other alternative would be to get a thread chaser and clean out the bolt hole where it originally went and reuse that mounting location.
Until the battery gets charged up, it will be toward the "+" side of the gauge. It's not uncommon for Chrysler products to show a slight "-" when in gear at idle with the accesssories (like a/c, headlights, blower motor) turned on. A larger capacity battery might help make that less of a problem, as I discovered on another car I own. You might also consider running a ground cable from the engine to the chassis too, even though Chrysler might not have done that from the factory. If you don't already have one, a self-regulating battery charger is a good investment too.
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