I've been trying to solve an overcharging situation. The alternator belt was squealing and after tightening it up a surge of voltage came through and burned up the inverter. The alt tested bad. Replaced alt and regulator. Then the battery started to boil and swell. I though it was going to explode. Replaced the battery, alt and regulator again. Same problem. Overcharging to 19 volts and even burned out both headlights. Replaced it all again to included new headlights. Going down the road, the headlights would flicker, go dim then to very bright. I check my trans fluid level at night and saw the trans dipstick tube sparking again the body. Obviously, the transmission is getting full voltage but from where? Had the alt tested it was bad. Replaced all of it again! Redid grounds, block to frame, battery to block, alt and regulator grounded. Turned on headlights and they blew immediately. Check loom between alt and regulator, all seems good. Still getting 18-19 volts. Then with the ignition key off I can take my test light and check the body, frame, engine block, radiator, etc and no voltage. Turn the ignition key on, not started and it's all hot. This is really starting to wear me out.

 

The ground between the block and the cab is missing. On a V8 it is from the back bolt on the intake manifold, but I don't know where it comes from on the six. Anyway, the other end is to a screw on the firewall.

The reason I say that is because the tranny is grounded to the engine and you've fixed that. But when you shorted to the cab you provided a ground to the cab and drew a spark. And, since the regulator grounds through its mounting screw to the fender, which is supposed to be grounded to the cab, your regulator isn't getting ground.