I have another take on gussetting, which isnt used much, as its a bit tricky, but adds substantial strength without external bulk or welds crossing the joint. Its a little hard to describe without pics..but here goes.. make a template the same size as the section you are looking at, but 3/8 taller above and below the rail. now looking down at the rail from above, you drop a cutoff wheel thruogh the joint in the middle of the rail, and do the same at the bottom from below, make it as long as your elbow shaped template,and only slightly wider than the material you use for the plate. drop the plate through the slot, and weld it in around both sides of the 3/8 section sticking proud of the rail. You now have three vertical surfaces carrying the load instead of two, and you havent created stress points across the welded up joint you have already made. I totally understand if this makes no sense, its really hard to explain! maybe if I mention its a technique used to put a lug into the end of a tube for aircraft control rods it might ring a bell..
My 2 cents. The plates will add strength at that spot. Your weak point will be one inch away from the weld in the tubing itself. I would make the cut at the joint curved instead of straight.
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