I'm a machinist and there isn't any metric threaded rod that measures 1.75mm. Are you sure the folks at HD didn't say M12 x 1.75? Inch threads are designated by the major diameter and the number of threads per inch. Metric threads are designated by the major diameter and pitch (distance from crest to crest).
Your tank hold down rod is either a 1/2-13 or an M12 x 1.75 (not likely). The next common size metric is M14 x 2.
Save yourself a bunch of money and just buy a 3' piece of 1/2-13 "all thread" and some matching nuts. Drill out the hole in the tray if you have to and cut the rod to the proper length.
The charge out rate for most machine shops in my area is $75/hr.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.