I don't worry about seams either, unless there's some aesthetic thing (looks) - generally I follow everything Oldoak mentioned.
I've built some hitch stuff similar to your project, what I do is precut every piece, then HARD clamp the tubing down to the 1" thick weld table at both ends and the middle, then I use a piece of 3/4" allthread and several nuts to string all the drilled pieces on - adjust the nuts for desired spacing and snug them up while the individual pieces are resting on a flat surface.
Then I check spacing at the holes, and at the welds, get everything lined up, then light tacks (2 per piece, 3 per gusset), then clamp the allthread nuts down HARD (by now everything's sitting where it SHOULD be)
Most of my clamps are rated 1200 pounds, and I'm pretty sure I'm there (unless the tube's too thin-walled to take it) -
Then I "hop-scotch" around til everything's full welded, and find something else to do til my IR gun says it's all at room temp. After I take off all the allthread stuff and unclamp from the table I run a straight edge across everything (and an aluminum rafter square), looking for daylight.
If THAT test passes, great; otherwise, Oldoak covered that too...
(Some say I'm too OCD; others are SURE I am) :laughing: ...Steve