Great minds think alike, I did that about 2 hours ago, hehe.

It wasn't hard to do because there are a set of bolts through the beam and the strap just inboard of the hulls. Undo these, insert spacer, fix spacer in with bolt (in my case), and tighten the tensioning bolts back up. Worked like a gem, got about 5mm prebend in the beam now.

Atleast the strap is under tension now before the rig tension comes on.