The original K-mart axle worked OK and it was a "U" shaped channel 1.5 inches tall and 1.25 inches wide. Two 2000# spindles with 1" wide round ends were welded into it. It worked OK from a bending strength perspective for over 30 years. The wall thickness was 1/8 inches.
The 3500# spindles are used for the purpose of upgrading the bearings. The trailer will continue to have a 2000# gross weight limit. The 3500# spindles had 1.5 inch wide square ends that fit inside 2" square tubing with 1/4 inch walls. That axle was massively heavy. Some beam loading calculations said that the tube could be lightened and still have the strength of the original axle. So, it was hacked up with a Sawzall to the configuration shown.
Thanks for the suggestion about the galvanized paint.