the speedway pal hubs are the answer for wide wheels, but you'll also need the drop down mounts shown at the bottom of the page RcMonk posted, otherwise the turnbuckles will foul the top deck.
For 26mm wheels you will be able to get away with thin spacers, too thick will mean there's not enough thread for the wheel nut to secure to. HPI used to include some very thin round spacers with their wheels, these are very handy if you can find them. I use them on the rear a lot because running on wood I use 21mm wheels and tyres and Schumacher are about the only company that make 21mm wheels that are easy to get and they have thick spokes that foul the suspension arms a little.