Cutting a long, custom made beam is a bit like formatting a long used hard drive. You never feel certain that you do the right thing. If a cut goes terribly wrong you are in trouble because the material is not retainable anymore or only at a great loss. So preparing the cuts is as important to me as the cutting itself.
I had done something like this before I actually ordered the material to optimise the beam length for the best price/length ratio. The longer and fewer beams the cheaper the whole lot with the constraint to meet the minimum quantity and a maximum length.
I use 6082 alu, which is a “structural aluminium” and not an off the shelf item in the size I wanted. It is generally difficult to find structural aluminium with any profile. I happened to find 76x76x6.35 also in 6082, which I use as an additional rail under the main, which increases the height of the two main weight bearing rails to 9″.
Overkill ? Yes, maybe .. I did run simulations on the chassis and the 6″ rail did not fair too well under the load including the safety margin. Also to consider is the length. Most chassis are shorter and max out at 3.5t and use the 152x76x6 as the sole main rail. I intend to have the strength in the chassis to go over heavy corrugations and not risk enough flex in the chassis to create stress cracks in the body. One of the reasons why I do not glue the floor above the neck to the chassis. The neck can deflect without stressing the side walls.
I have an average of 20cm cut off from each 9.9m beam to allow for recuts, especially for the angled ones, in case something goes wrong. If I would build a second trailer there would be only some dust left because even these cut offs could be reused for another trailer.
The steel vs. alu discussion very often contains the argument that alu is not that much more efficient in weight because one needs more wall thickness to achieve the same strength as steel, which makes the weight advantage negligable. This maybe true for a smaller trailer but surely not for one of the length I am building.