So I took the failed tach apart. It already didn't work so I couldn't make it worse. I was hoping for a dead battery. No luck, it was potted. Aimed it towards the trash and missed. Twice.
Anyway, the new tach is here. I bought the new fluids today. Most of those I was able to find at WallyWorld but had to order the special transmission oil from Amazon.
It's going up on jack stands Monday and getting a thorough going through. After finding out what the guy my Dad paid to do, or didn't do, I have to.