I have the northern tools 5×8 trailers and I just received the 15″ wheels/tires that I ordered elsewhere. I expected the fenders to interfere with the larger tires, but I was disappointed to find that the sidewall interferes with the heads of the bolts that hold the leaf springs in the spring holder.
a friend suggested I purchase a set of spacers (off-roaders use them when they install huge tires) to move the wheel an inch or so outboard.
my only concern about this approach is possible negative effect on the bearings due to the wheels being 1″ further outboard than designed. I purchased a set of “high-speed hubs” from NT and they are rated for 1250 lb/side.
the only other idea I have is to have a welding shop add 2″ to the center of the axle.