When I built my new trailer, I laid out the spring hangers first, prior to anything. Triangulate the hangers to the hitch ball, particularly the first one on each side. If you buy new axles, and have to weld on the spring perches, locate the brake mounting pads equal distance from the frame rails, left and right. That will center the axles. The spring hangers will locate them front to back. My trailer tracks nicely. The build is documented in my gallery, and here.
Trailer Build (http://www2.***boat.com/forums/showt...oject+hawaiian)
it came out nice too. good job!