I had a big 39 plate 6 Volt battery on a tractor explode on me when I was 17. It had been charging for a few days and I had built a super strong angle iron 'cage' for it, probably saved me, my spanner touched the cage on the final bit of tightening and I learnt the very valuable lesson that the Earth gets connected last.
I was blown about 30 feet along the workshop and hit the wall between two steel uprights about 10 feet above the workbench. Was functionally deaf for a couple of weeks, luckily I was wearing safety googles and ear muffs at the time. The imprint I left in the corrugated iron wall stayed there until the building was torn down.
Years later I saw someone blown clear over two B-trains parked side by side. I treat batteries as if they are loaded bombs.
Very glad you have survived and I hope there will be no lasting damage. Wishing you a speedy recovery.
I have done it a thousand times and I know that you should put a minus on the chassis. But old cars rust everywhere Then it is easier to connect directly to the battery