Some info
1boats are heavier than they look.
2 if you are putting breaks on it. Electric over hydro. Are the way to go.
You can even get disk breaks for extra .
You didn't say what kind of boat. My 18'Lund deep V came in at 4300#
Now that was with a 125hp out bord. And full of fuel. 2 anchors, trolling motor.
If the axle is a 3500# the mounting flange should be welded on. So all you need is the breaks and drums. And wiring. And the break controller.