I put my glasse's on and read that twice .
I'am some what puzzled by several thing's
1.) 6.5 beam ??
2.) 250 ton ( as in 500,000 lb's )
must be a typo .
I build in Alum and steel , and have for many year's , might I suggest you define your need's , then look at different design's and designer's , to find something that will suit you . Bear in mind , learning to weld is easy , learning where to weld is not , having a weld schedule is part of a good design and I would not bother to build a " tin-boat " without it .
Working by your self , steel has one disadvantage , it is heavy to work in big pc's with out material handling eqt. ( ie cranes , forklift's , gantry's etc. ) and you want to use the biggest pc's possible . Alum also has many advantages , the best of which is ( all thing's being equal ) alum will have a higher resale value .
BOL