I would keep the bunks over putting pads on it. If you need taller posts or want to do welding on that trailer keep in mind it looks like the steel
on that trailer could be hot dip galvanized, and if it is you do not weld where it is galvanized. If you need an explanation of why that is dangerous be sure to search that out.
Additionally, prepare to do some serious work
on the wheel
bearings by replacing them or at least repacking them with grease. I just had a buddy of mine ignore that advice on a road trip and between the roadside tow and the weekend shop rates he got gouged for $3K for his trailer. Even the tires had to be replaced, they were junk. Don't be that guy.
I've had a lot of trailers and still currently own 3. The first thing I do before I take a new-to-me trailer on the road is to do a thorough inspection
of the wheels, tires, and brakes. If nothing else at least have those checked out before you load a boat
on that trailer. An empty trailer can be towed a reasonable distance without doing that, not a fully loaded trailer.