Wanna save some trouble and time? Just go ahead and get "beams" longer than you need, then cut A SECTION FROM THE MIDDLE to make them the right length using a die grinder or a dremel with a metal cutting wheel then take them down to have them welded at a machine shop.
Or, you could just have them mess with welding rivets, but that's going to be expensive and the point of failure. As an amature welder, I can almost guarantee two things... 1) these beams are "low carbon" steel and are more prone to bending instead of breaking, which is easier to weld but doesn't have near as much strength. 2) fitting a mig head or tig head under that rivet is going to be tough. I'd go ahead and drill from the back and weld them in that way, but this would raise the carbon content of the rivet (the point of maximum pressure)
I guarantee your better off having a fella weld a straight seem like that...especially low carbon steel that has been prepped (as in it's clean of paint, rust) with a wire wheel before it makes it to my shop. If you bring in a bunch of these beams at once, and they're all prepped properly the job should cost you a grand total of about $2-$5 a piece, depending on how busy the shop is that afternoon.