I did it with my single axle. Axle Inc. Move up to next size 3500#. HD them do all the work. Weld axle mounts and spring alignment and mounting, etc.. Just jacked up the trailer slid the new setup in, 4 bolts, Done! I needed a friends business to ship it to. Cost an extra $50-$60. They have a form for measurements on the site so you can order perfect. I used 3500# axle with 2 1500# springs at their advice to cut down the bouncing. Works great they know the formulas.
They really did a good job. All in was around $320 for the axle and springs mounted.
20 minute job. No welding