Long weld

Hit there!............Still working on my ramp, weekend work very slow ...........I would like to know from the experts out there what would be the correct process to weld a 4m long flat bar 60x5 perpendicular to a 4m long RHS 150x50x3 duragal. I have to make 2 of this things. This flat bar will be supporting the deck made of 240x45 M12 H3 pine........... I was planning to do 100 mm weld and then 200 no weld on both sides but I spoke to the galvo people who will do the hot dip and they said that in order to guarantee no rust it will have to be welded all along with no gaps..........So all weld it is. I have already tack welded both RHS pieces together to eliminate bending and will start welding the flat bar (one to each of the beams) .......... I would like to know the best way to avoid it pulling to the side of the weld and keep it square...............your replies are appreciated, sorry for the monolithic block of print, but this server at work does not recognises return nor does it allow me to edit.:D