I'll presume you have mobile home type axles where the lug bolts screw into the hub with wedge spacers that center the rim since your description is lacking, but your solution alludes to these axles.
If so hardware store thread rod may get you going and is very ingenious, but likely grade 2 rod and not proper for the application.
If the bolts don't need a tapered shoulder (they mustn't since you welded a nut to a rod), then just use grade 8 bolts.
If they're shouldered (self centering) then you have an issue with centering if not using a cone shaped taper shoulder bolt.
Good luck with your project.