Since it had to be welded on and I'm not a welder, I paid a welding shop to do the job. They cut the original plate off with a grinder & cutoff wheel, and then changed to a grinding disc to smooth it down to the rail. I would've preferred to leave the original plate on and weld the new one next to it, but that would've put the jack just a tad too far forward or rearward.
They make a weld-on plate for it with the mating holes already tapped, etc., but it's sold separately. My trailer shop had to order it from Fulton and wait 2 weeks for it to arrive. Best bet is to contact Fulton directly and ask them the best way to get one. You could also make your own mount plate but it wasn't worth it to me to go to the trouble.
Here's the new plate after welding. I painted it before bolting on the jack.
