I had to sift through your posts to verify your year. You’ve got a 1954 with separate gauges, right? (you should put that in your signature block) ADDED: Then I see you did!
To me, a spark like that indicates a circuit. Something was connected and that caused a spark once the battery was added to that circuit. Are there any other wires at the positive terminal, i.e. fog/driving lights? Fused?
Depending on the damage, the battery post could have severed (melted) inside the case, or you just took off a bit of surface on the terminal.
The original jeep wiring couldn't be much simpler than it is. The only "fuse" is the circuit breaker on the back of the light switch. How long did the reversed battery stay connected? Again, to me, if the key is off, then there shouldn't be any electrical flow to the coil and the distributor. That should only happen when you switch to "on" for the start. But a good check would be to look under the instrument panel. The power routes through the Ammeter to the switch. Are there any wires touching another terminal or missing insulation against metal?
Is the starter original or aftermarket with a solenoid? In the past, have you ever had issues starting?
If I were doing this, I'd try a spare battery in the jeep (if you have one). If not, WEAR EYE PROTECTION (I've had a battery explode on me and it hurts), suspect a disconnect inside the battery until you prove otherwise. The chemistry inside the battery produces hydrogen gas - which itself is flammable. I'd check the battery to see if it has at least 12V. Wiggle each terminal to see if there's any play. If either of those tests don't pass, get another battery. Next, I'd connect the positive first. Then tap the negative cable end to the terminal. If no sparks, connect the cable (you can connect remotely, i.e. jumper cable to the frame but if the severe is inside the battery it won’t matter – this is recommended when jumping a battery).
What is the ammeter showing (center or discharge)? Another quick peek under the instrument panel at the wires. Turn the ignition switch on to see if the gauges work, i.e. fuel and a little amp wiggle towards discharge. Try a crank test.
If nothing, turn on the lights and try a start again. If the lights stay steady with no crank, there may be a connection break to the starter. If the lights go real dim, I suspect the starter is dragging and will need rebuilding/replacing.
If you get a crank, hopefully it starts. Watch the ammeter to see if it starts with a discharge and jumps to charge (above idle). If so, I’d suspect the regulator is still working.
Remember the eye protection. Can this be done outside of your garage? I’m thinking about acid spray in case the battery does explode (worse case). Outside you have a hose to dilute and none of your other garage items get sprayed.
p.s. If it does explode, hose down everything. The liquid is sulfuric/water (electrolyte). Did I mention it hurts?
