Gazza i used a piece of 100x75 for my draw bar. Got it from a mate who owns one of the brand name trailer places, he says thats plenty. Boats 2.6t. I used mostly gal bolts after checking the galvanic scale and price. Use plenty of a quality anti corrosion compound. Put a gal subframe under the i-beams. 6mths now, no sign of electroysis and just put 7000klm on it with no probs. Solid as a rock.
Mark