pghportager,

At the risk of being too prescriptive, I learned a few lessons:

1) Weld up the cross trees first.  Then slip the "sockets" on the uprights (use some tape to hold them in place), clamp the whole thing to the trailer frame, and tack the sockets to the frame.  Remove the cross trees to finish up welding the sockets to the frame.

2) No matter how careful you have been, the cross trees won't just slip into the sockets.  The heat distortion won't let 'em.  So take a scrap off-cut, put some light cutting oil on it, and then dust it with course lapping dust (if you have course lapping compound that should work fine).  Work the piece of scrap up and down in the sockets until it runs free.  You'll probably need a hammer and a couple of hours to get it really cleaned up.

3)  Use an angle grinder with a sanding disk to really clean up the lower part of the uprights to bright metal.  Then put some grease on 'em and see how they fit.  My guess is it will still need some wiggling... mine did!

I took a few photos of details and can provide them, but this board probably isn't the right place (they're half a meg each).   Send me a PM with your e-mail address and I can provide anything else you think may be helpful.

WS