suburban furnace tripping on high temp

How long does it take for the Hi Limit to trip? Have you checked the INTAKE from outside to make sure there is no restriction? A partial restriction would cause lack of some air to the burner, but usually this will cause a rich condition and black sooting. I would suspect a bad motor, not running at correct RPM's, but enough to make the sail switch close. With inadequate volume of air, the furnace burner casing will run hotter. The reason I suspect the motor is you stated that until now, the furnace ran correctly even before you attempted the ducting modifications. Doug