The old battery in my 1970 SS 454 El Camino got to a point where it was overheating, acid would spill out...terrible smell, etc. Replaced the old battery with a brand new battery. During short drives no problems or issues. Went for a 45 mile drive, maintained constant RPM (3500) for 20 plus miles. When I stopped the car, I could smell acid boiling inside the NEW battery. The smell was terrible. What could be causeing the battery to overheat & overflow acid?

Check the voltage regulator ... It is probably over-charging ...

Explain why....what should it be

Your car, unless altered, runs on a 12 volt electrical system. If you were to connect a volt meter or multi-meter to your battery ( engine not running ), you should see 12.x volts on fully charged battery. The battery's main funtion is to start the car. The charging system should supply all power needs for a running car ... When the car is running, you will generraly see more than 13 volts ...a good system may show somewhere between 14.2 and 14.5 volts ... Electricity equals heats ...The voltage regulator "imposes" an upper limit on the voltge produced ... When a voltage regulator goes bad, you may actually see as much as 18 volts or more .... This will cause electrolyte/acid within battery to boil ... BTW, also inspect the quality of your cables ... A bad cable will create additional resisitance which also leads to heat ...

Thank you for your post. Now I understand completely. I really appreciate your help!