Dirk,
I understand your desire to raise the rear beam, I get a lot of waves crashing into the rear beam, slowing the boat noticably when it's blowing over 15 to 18 knots. Could probably be reduced by moving crew weight fwd, but those are the conditions where I constantly have an eye on the leeward bow (I have very gusty conditions usually), and I feel most secure being on the wire, fully aft with a foot in the footstrap. So I'm not moving fwd, if I have skilled crew (not that often), I'll have them moving fore and aft, as well as in and out, trying to balance keeping the rear beam away from the chop, with the strong desire to keep the lee bow visible.
It seems like your design really only raises the rear beam 2", and I wonder how much difference that will make. But your design seems sound to me, what about doing all the welding on the half round section only? Weld a flat rectangle to the half round piece, and attach that piece to the square tubing with bolts and a backing plate inside the tube? That would obviate in need to do welding on the square tube itself. And have "U" brackets made of ss to hold the assembly firmly in the cradles, similar to the original clamping brackets. Seems like this would minimize any weakening of the square tubing from the welding.
How will you attach the tramp to the rear beam? Will you use the original main traveller set up, or a new conventional traveller track? What will the material for the front and rear beams cost you? Where are you?
I hope you'll keep us posted on how this shapes up. I'd like to do it to my boat someday, and I'm sure there are others out there.

Dave

SC20 and Flight Risk