First, if you have a way to get it to you, I think I have have a rear 5.5 crossbeam with a dent in it. It is in Houston. I may have some other cross beams that you can have. There are probably people like me near you. We held onto damaged hardware first, thinking we might need it and later because the snakes living in it had gotten BIG.

A couple of comments:

The front beam does not look like the one on my 5.5 uni. The tramp track on your is separate, mine was internal to the beam, like the 6.0. It may be a homebuilt replacement. I would not order anything based on the dimensions of that beam. Also, I do not remember factory beams corroding like that

I have replaced at least 3 NACRA crossbeams. Online companies normally only stock 8 foot beams and shipping an 8.5 foot tube is over half the cost. Your best bet is to find one locally.

A used 6.0 beam with no corrosion would be ideal. Check other catamarans for beams that have the same dimensions, F18's for example.

Did the Inter 20 use the same extrusion?

If you buy a tube from a metal dealer, tell them this is is for structural use. No reputable supplier will sell you anything they have questions about. I had an industrial supplier search the stack for a tube that had 6061-T6 printed on it and the paperwork taped to it. Try to avoid surplus companies, they do not always know what they have or you may be buying a reject.

Good luck