Go thick. Because the sheet dissipates heat so efficiently in the plane as opposed to in the cross section, it's stupid easy to pucker the stuff. I had a bunch of not-good results with 16-ga; I did marginally better with our backwoods buzzbox TIG (might have done hella better if we had a pedal and were doing more than Sears Roebuck buzzbox plus arc stabilizer plus argon bottle and handpiece).

But there's definitely something about welding with baling wire and old coat hangers. It's liberating. Eighth inch plate is like woodworking.

As to your torch question, that tiny lick of flame better go out after a few seconds. If it doesn't hell yes change out the seals. I've never dealt with an acetylene explosion but they do a real good job of teaching you not to fuck around.

BTW is it a riding mower? 'cuz yeah. I don't have a lot of experience with riding mowers, but the one I converted to electric had wretched build quality on the steering. Like, the thing was three years old and the teeth were mushroomed. I swear it was made out of roquefort or some shit. I halfway wonder if that's their garbage-ass way of putting some shock absorption into the steering; use something with a young's modulus close to polyurethane and if it turns to cheese who cares because the dumb thing only has a 3-year warranty.