Hot battery cables with New battery, what's a possible cause? Options

[no1uno]

I'm trying to help my father get a project 914 started. It hasn't been started in a year. We tried to install a new battery over the weekend. First, we installed the positive terminal. When we went to install the negative terminal there was a larger than usual pop/spark and a sizzling sound. Within seconds, it began to smell and the battery cables themselves got very hot. It's as if there is high, ongoing load as soon as the negative connection is made. Any ideas? The fuses under dash look fine. The blue 25amp on engine relay board was blown. I replaced it but had the same problem. A bad ground, perhaps? What could draw so much power so quickly?

Thanks in advance for your advice.

[442nd914s]

If its all original wiring, I would start by:
Disconnect both terminals at the battery. Remove battery
Continuity check as follows;
Negative terminal to a few ground points. This will verify a good ground terminal connection.
Disconnect all 2-3 (possibly more) red/accessory cables from the possitive terminals. You should only have the possitive terminal remaining. Disconnect terminals at the starter. Now all these disconnections, you can do a continuity check to ground. Check from ground to all positive wires, your results should be all open. Which ever wire gets u a "close/ohm” reading, THATS wire is shorted to ground. Follow it with a small grip to check for chaffing or possible strans protruding.
In my experiance with my 73, the positive terminal at the starter missing its cover and bent so tight it was approching disaster.
Wouldnt hurt to check your battery since you have the meter out. Possibly a bad cell or missed marked post. Most auto parts stores check batteries for free with a load on it. This is more acurate.

Good luck