I converted a trainer to a taildragger some years ago and simply installed an aluminum gear so that the wheel's axle was just a small bit ahead of the leading edge of the wing and installed a tailwheel. Obviously you need to reinforce the area where the gear will be attached with plywood and triangle stock between the plywood and the fuse sides and/or formers.

Make sure that the wheels are well aligned by making sure that both wheels are the same distance from the rudder and nose.