Unfortunately in order to get the blindnuts inside the fuselage where they belong to hold the wheelbracket you will need to cut away some covering to get inside.

An alternative I would suggest is to screw the tailwheel assembly into the fuselage rear with regular wood or sheetmetal screws. Probably #4X3/4 screws would be perfect for securing the assembly onto the fuselage. The sullivan wheel assembly has 4 predrilled holes for attaching so feel free to use as many screws as you feel like using. It would be more secure if you had a lite-ply doubler at the rear where the bracket is gonna screw into but if it is not doubled no big worry (just a little less secure) so use about 3 #4X3/4 screws to hold the bracket in place.