I have shortened more than a couple of these frames but am no way anywhere close to any of these professional welders.
Will this frame fail, more than likely not. Looks fine to me.
Would I plate it anyway, H*ll yes. Why not. If you don't want it to be seen fish plate the inside.
Really though, most of these trucks lead somewhat of a pampered life once they end up in our hands. We use our Diesels for heavy hauling.
OP, carry on sir, you are doing a fine job.
Those pointing out potential weakness in the frame butt weld I agree. For most guys (me included) this method should not be used. Leave it alone (or to the professionals) and use a step notch. I even plate these. Cheap & easy insurance for me.