I many times switch battery switches to "all" to get plenty of spin power when starting. Your comment about switching to 00 cable makes good sense too.
I frequently hear of overheating issues on boats, and people go after the usual culprits and that usually fixes the problem. You know, the impellers, drive belts, pump cam, thermostat, cleaning out the pressure valves to be sure water isn't short circuited during low speed running, and checking for air leaks in the connections and pump itself.
As you can see from the photos in the following exhaust manifold thread
http://www.network54.com/Forum/424840/message/1179060460 some or all of the overheating issues can come from the low grade iron used in exhaust logs flaking off and forming an obstruction.
Since this is such a tough job, wrestling with the heavy pieces, I would absolutely recommend getting a pyrometer, which can be used on your car and home AC/heating system too, and to check your circuit breakers in the middle of winter to see which ones are taking the heavy load
http://www.network54.com/Forum/424840/message/1136482174/
On the offending motor, I would run the pyrometer all over the motor looking for hot spots or signals of obstruction. Thermostats, hose inlets and outlets, transmission oil coolers (a great place to form an obstruction in front of the tubes) and pressure valves all play a part in the 427 system. (If I recall, you are running 427s too).
If you have water flow and your'e still overheating, it is possible the water is taking an alternate route on you and still giving you a hot spot. Here is a relevant thread on the cooling issues:
http://www.network54.com/Forum/424840/message/1123102009/
Pressure valves that are stuck open with debris will cause low speed overheating. High speed overheating can happen if you have an internal obstruction somewhere causing pressure within the system and premature opening of the pressure relief valve: If that happens you are just dumping water out the pressure relief valve. It may be possible to see the motor getting hot if you are able to let someone drive and you constantly monitor temps with the pyrometer. If the whole motor gets hot, the reading will be hot everywhere.
Good luck!
Regards, Paul