My axle (Dexter #9) is rated for 2200 lbs. I estimate that my TTT will have a "dry" weight of under 1300 lbs, and no more than 1500 loaded for camping. That gives me a load capacity right at the 50% over loaded weight that Jack mentions. My TTT will weigh 50% more than most TDs, so you surely don't need anything heavier.
I did wait until I had my Dexter axle before cutting the steel for my frame and I'm glad I did. Despite being told differently, I ended up with only 1/4" clearance between the swing arm and the frame, which would not have been enough if I had cut it for 60" width. Since my 1/4" plywood will extend down the sides of the frame, I needed an additioonal 1/4" on each side to clear the arms without having to make cutouts in the plywood. So, I cut my frame for 59 1/2" width to compensate for the needed clearance. Losing the 1/2" is no big deal, since I'll have my mattress cushions custom cut anyway at
Foam Products an hour's drive from my home in central Florida.