So you have 3 x "leg kits" with 4 x connecting rails across the three legs. ( x 2 for the other array)?
The connecting rails are joined to extend them across the three legs with connectors.
You only need to buy 3 sets of U brackets as they come in 4's
Seems to be all there although we ordered the same kit and did not receive 256246 so had to bolt the upright legs which is more secure anyway. When I complained I got another box of 249770 which you throw away anyway and never got a refund or the correct parts.
Also they can send badly marked ally sections so check these are not water marked where they been sitting in a yard somewhere. Our stock came half from UK (old stock badly marked) and half from Germany.
Check everything on delivery because they will not respond to requests and only deliver once a month.
Also if you are looking for end caps on the rails you will not get any as they do not exist.
You have not ordered any cruciform kit? This stops it moving sideways so any reason?
Eyeing the structure was the best way to ensure all the diagonals were parallel but we had 9 leg kits in a single row.
I have a box(large) of 249770, 1 x 249746 long horizontal 6m rail, Lots of 249768 clamping plates.
We used ground anchors, for the record, and so no time at all to put the bases down.
Good luck.