Serge,
<br>You've heard the term "patience is a virtue" well my solution would require some of this.
<br>The rear beams I've built up in the past that consisted of a seperate "I" beam on the aluminium rear beam were all bolted.
<br>The bolt goes through the "I" beam and top surface of aluminium rear beam and a washer and Nylock nuts goes on the bolt inside the beam. You get the nut and washer in position by taping a spanner to a batten. Put a bit of packing tape around the nut before putting it in the spanner and the extra thickness holds the nut in place. Use Duralac to make the washer stick to the nut and prevent a reaction of dissimilar metals between the Aluminium and stainless. Start from the centre of the beam and work your way out each side.
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<br>Get a frriend and a couple of beers and take your time and you'll never have to touch it again.
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<br>Hope this helps.
<br>Phill<br><br>