Buy the perches for 2.5" axle tube and modify slightly to fit your larger tubes.
I recently did same for my 14 bolt with 3-5/8" tubes and purchased 3.5" perches (1/8" difference, I might have actual measurements off). When I get a chance I can link a picture or two, until then look in my photo link below and folder with axle stuff.
You can do a nice enough job with a paper template and an angle grinder or file, it's not much material to remove.
