-2 is typically used in TIG welding where the material is or should be very clean. -6 is usually used for MIG wire but is available in precut TIG wire also. -6 has a higher silicon content which helps pull impurities from the puddle when the material is slightly contaminated. Other than that they are pretty much the same. Some people like the -6 for TIGing. I use -2 since it is the most widely recommended for clean 4130 which is usually what I do. I don't do much "dirty" stuff.

I would check the tubes with an alignment tool before welding it up, just to verify the thing is not bent before you do anything. While you can put the ends on to match the differential centerline, if they are way off, it creates all kinds of other goofy problems with the wheelbase being moved, etc. Mostly on sheetmetal housings, but I would still look at it.