I believe this is incorrect. If I remember correctly, transport was not the only issue with building the solid rocket boosters in one piece: physically pouring a solid rocket motor of that size was infeasible.
http://www.cedengineering.com/upload/Ethics%20Challenger%20D...
The point being that if the rockets were welded and fueled near the launch pad there would have been no need for o-rings. The reason for o-rings was that cylinders full of fuel needed to be shipped around and welding them together would have meant too big a lump of explosives. Casting a big engine had nothing to do with it.