Yes, by turning that threaded post a quarter turn, you effectively "scrubbed" the two contacts together and hopefully wiped off the oxides that were preventing good connection. I don't know if that would be as effective as cleaning the contacts with a piece of material or not, but you'll find out.

One word of concern though... That threaded stud is a calibration device. More pressure on the switch and the gauge runs higher. Lower pressure, the gauge runs lower. I don't know how much of a change in the needle position will result from a quarter turn. Might be noticeable, or it might be so slight as to not really matter.

Now that the oxides have been scrubbed, you could probably turn the screw back to where it was to start and the connection would still be good. Do it four or five times and call it a day (if you haven't already)

Oh, and sorry, but I got nothing for your heater cable. Never messed with the 240 stuff.

Good point on the calibration.  I'm thinking of taking out the sending unit, hook it up to the gauge and manually move the float up and down to see the effect on the needle.  I did put some gas in the tank and am not sure if it's above the level of the sending unit.

Well as I said, I'm not sure how much difference a quarter turn would make, but just wanted to mention it.

As for calibration... Here's a pic of some testing I ran some time ago. Ballpark should operate something like this: