Half Torsion Axle Mounting Question

Hi all. A newbie here with what's probably a silly question. I'm been bitten by the Teardrop building bug and am in the process of considering designs. One I'm particularly interested in is Angib's Superleggera (http://www.angib.pwp.blueyonder.co.uk/t ... tear32.htm). I'm interested in this one primarily due to it's light weight, as I'd like to tow it behind my Scion xB. I really like the style and simplicity too. I've priced out having the steel frame constructed (got a trailer shop right around the corner) and am now looking into the suspension/axle units. The plans call for what looks like a half torsion axles. The ones I've been able to find at outfits like etrailerpart.com look different than what's on the plans. Angib's drawing shows a unit that's fairly compact - I suspect it's spring powered. The unit's I see on this side of the pond are all kinda long - the 550 lbs axles I need are 7" long with 3 pairs of mounting holes along the length of the body plate. So my question is, how are these axles secured to my 2" axle tube? The guy at the trailer shop said that they just U bolt onto the square tube. Is that right? Or do I need to have the shop weld mounting plates to the trailer's axle tube. Any guidance would be appreciated.
Thanx,
Thanx,